Intuitively, it's easy to accept this configuration without questioning it for even a second, because we are all familiar with freeways, where slower traffic keeps right and faster traffic passes on the left. (photo credit: mojoey) That paradigm is so deeply ingrained in our minds that we tend to think high speed trains should obviously operate the same way, right?
Not so fast-- with nearly every Caltrain station from San Jose to San Francisco about to be totally redesigned from a blank sheet of paper, it's worthwhile to question all assumptions, and in particular the assumption that HSR should run on the inside pair of tracks, i.e. slow-fast-fast-slow when enumerating the tracks from one side to the other. (Credit to Richard M for raising most of the following ideas in previous comments).
What if Caltrain local service ran on the center pair of tracks, with HSR and express trains on the outside tracks, i.e. fast-slow-slow-fast? What would be the pros and cons of each approach? For HSR? For Caltrain? For communities abutting the tracks?
Anyone who commutes on Caltrain knows that periodically, an incident occurs that puts one track out of service for a few hours. While it is tempting to ascribe this to grade crossing accidents and equipment breakdowns, both of which would be alleviated by electrification and grade separation, the fact remains that service disruptions can and will happen on occasion.
When one track for local commuter trains is shut down, service is typically cut over to the other commuter track for a short stretch around the incident area. Caltrain has the option of switching tracks at over a dozen crossovers, spaced every few miles along the peninsula. Trains can temporarily run the "wrong" way and make their usual station stops on the other platform track. To the extent possible, this minimizes delays and inconvenience to passengers.
With HSR in the mix, it gets more complicated. If HSR runs down the middle pair of tracks, cutting over local commuter trains from one platform track to the other platform track requires crossing over both HSR tracks and thus waiting for, or delaying, traffic on those center tracks. Temporarily running on the "wrong" platform track would involve a complex, coordinated sequence of moves that disrupt service on all four tracks. In addition, waiting passengers would have to dash to the opposite platform in order to catch their train.
If HSR ran on the outside pair of tracks and Caltrain commuter service on the inside pair, a disruption on one of the commuter tracks would not conflict with HSR service. To switch to the other platform track, locals would simply cross over to the adjacent commuter track. Under this scenario, Caltrain stations would have a single island platform in the middle of the right of way, located between the center pair of tracks. Passengers would not have to switch platforms to catch their train on the other platform track, since the platform tracks would serve each side of the same platform.
What happens if instead, one of the express tracks is disrupted? It's not nearly as bad: a train can simply be routed to the adjacent track. Unlike local service, platform access is not required for HSR and Caltrain express services, which would stop only at key stations with four (rather than two) platform tracks. Therefore, the requirement to switch to the other platform track does not apply; any track will do.
In short, the fast-slow-slow-fast track configuration provides great flexibility for dealing with service disruptions on any given track. On the other hand, the slow-fast-fast-slow configuration causes a big mess that disrupts all four tracks, whenever one of the local tracks is knocked out of service.
One other interesting fact is that in certain locations along the peninsula, Caltrain service could theoretically run in both directions on a single track without trains ever conflicting with each other. (Hello Atherton! Take note!) This naturally depends on the density and scheduling of Caltrain traffic, but it opens up the possibility of having only three tracks in certain locations, as dictated by the operational service pattern (Hello Atherton! Service pattern, not your back yards!) Switching to a three-track configuration from a four-track configuration is much simpler if you go from fast-slow-slow-fast (4) to fast-bidirectional-fast (3)... a simple turnout, and presto. On the other hand, trying to neck down from slow-fast-fast-slow (4) to any combination of 3 tracks invariably requires expensive flyovers to avoid frequent fouling of HSR traffic by local trains.
Operational Flexibility - advantage: fast-slow-slow-fast
Turnbacks (added 18 Jan)
The option of turning some commuter service back in the other direction before reaching either end of the line provides additional flexibility to tailor service patterns to passenger demand. This falls under the broad umbrella of operational flexibility, but merits a brief mention. For example, some Caltrain service could be turned back at Mountain View, heading back north where most of the ridership demand currently exists. Similarly, future Fremont service could be turned back at Redwood City. When a train is turned back, it needs to switch from one local track to the other local track. In a slow-fast-fast-slow configuration, this move requires fouling all four tracks in both directions. The fast-slow-slow-fast configuration, on the other hand, allows turnback tail tracks to be placed in the center of the right of way between the local tracks, with turnback moves having zero operational impact on any other track.
Turnbacks - advantage: fast-slow-slow-fast
For the slow-fast-fast-slow track configuration, commuter (Caltrain local) stations would have two outside platforms, like the existing Bayshore and Lawrence stations. For the fast-slow-slow-fast configuration, commuter stations would have a single island platform, like the existing Belmont station.
A single platform has several advantages for passengers: there is no choice of which platform to use, making station access less confusing. A single platform is more pleasant, because it is typically about 30 feet wide rather than 15 feet. It is safer, because there are fewer opportunities to be isolated from other people. It is cheaper to provide all the station amenities, such as benches, shelters, ticket vending machines, elevators, information signs, platform lighting, etc. since they are shared for both service directions.
Ease of Station Access - advantage fast-slow-slow-fast
Platform Comfort - advantage fast-slow-slow-fast
Platform Safety - advantage fast-slow-slow-fast
Cost of Station Furnishings - advantage fast-slow-slow-fast
Impact to Abutters
On stretches of track between stations, both track configurations require about the same amount of land. The fast-slow-slow-fast configuration puts high speed traffic 15 to 20 feet closer to abutters' back fences. Whether this makes a material difference to noise levels remains to be evaluated, since high speed trains are generally quieter than any other train type at speeds of just 125 mph.
The fast-slow-slow-fast configuration also requires more land for the approaches to commuter stations, because the outside fast tracks must slew aside to clear the center platform in the station. On tracks built for 150 mph operation, tracks can be slewed by the necessary amount (about 15 feet) in a run length of about 1200 feet before and after the platform itself. These dimensions require an extra four triangular strips of land, 15 x 1200 feet, at each of four corners of the station area; in total, just shy of 1 acre.
Noise Impact - no clear advantage
Land Impact - advantage: slow-fast-fast-slow
In a slow-fast-fast-slow track configuration, HSR trains would run straight down the peninsula without care for Caltrain infrastructure. In the alternate scenario with fast-slow-slow-fast island platforms, HSR must jog around each and every one of about twenty Caltrain station platforms. However, comfort for HSR passengers would not be adversely impacted even with the somewhat frequent twisting and turning of the high speed tracks. The key to comfort in curves is managing the lateral acceleration and its time derivative (also known as "jerk"), and the dimensions mentioned above account for suitably benign track geometry.
HSR Passenger Comfort - no clear advantage
Union Pacific runs freight trains over Caltrain's peninsula tracks, to serve various industrial customers as far north as the port of San Francisco. There are roughly two round trips per night. Service to industrial branch lines could continue essentially unchanged under the slow-fast-fast-slow track configuration. If the local tracks were moved to the center for a fast-slow-slow-fast configuration, the occasional freight train would have to cross over the outside HSR track to reach the customer. Given the massive difference in axle loading and speed, it is unlikely that running freight trains over any HSR tracks would even be feasible.
Freight Service - advantage slow-fast-fast-slow
Impact to Caltrain Stations (added 02 Jan)
HSR on the peninsula will require the vast majority of Caltrain improvements made in the last decade to be demolished and rebuilt. Dozens of platforms, buildings, etc. will have to be moved to make way for four tracks. However, Caltrain has two stations, Lawrence and Bayshore, which could be used essentially as-is for slow-fast-fast-slow operations. Converting these two stations to fast-slow-slow-fast would require demolishing and rebuilding the platforms, modifying the pedestrian access, and realigning the tracks. On the other hand, the Belmont station is in the opposite situation: it could be used as-is for fast-slow-slow-fast operations, but would have to be entirely rebuilt for slow-fast-fast-slow. Whatever approach is chosen, there will be nearly total reconstruction of every Caltrain station, and the issues with Bayshore, Lawrence and Belmont will be lost in the noise.
Impact to Caltrain Stations - no clear advantage
Future Infill Stations (added 13 Jan)
Commuter stops that are added at a future date, as development fills in around the tracks, are termed infill stations. With a slow-fast-fast-slow configuration, they are easy to build by simply adding a pair of outside platforms and suitable pedestrian under/overpasses. With fast-slow-slow-fast, realignment of all four tracks is necessary to provide clearance for the new central island platform. Communities along the Caltrain right of way are already well developed, so future infill stations are unlikely to be numerous.
Future Infill Stations - advantage: slow-fast-fast-slow
To pull together all these pros and cons, it is useful to pause first and remember that Caltrain's most valuable asset is its land. As such, HSR ought to be considered a tenant on Caltrain's property, which means giving proper consideration to Caltrain's need to provide effective commuter service. That is why operational flexibility should be weighted heavily. Putting all the factors together in a table, and assigning weights, one can build a trade study to identify the optimal solution. The nice thing about a trade study table is that arguments about the best solution can be reduced to arguments about the factors, and the weights assigned to each.
One can quibble with the weights assigned to each factor, but the conclusion seems pretty emphatic: the Caltrain tracks should be placed in the center, flanked by HSR on the outside. In short, Slow Traffic Keep Left!