Elon Musk unveiled the result of his Boring Company’s work beneath the streets of Southern California—a 1.14 mile tunnel wide enough to admit one of Musk’s Tesla sports cars—which provoked BART into a Twitter tiff over the efficiency of his vehicles versus theirs.

As a publicity stunt, Musk staged a race between one Tesla roadster on surface streets and another through the new tunnel to illustrate what he says is the speed and convenience of his proposed underground traffic network.

According to the Verge, “The tunnel Tesla was the clear victor, emerging out onto the road a full three minutes and eight seconds before the one took surface streets.”

Touting his tunnels on Twitter Friday, Musk declared that the contemporary transit axes should be flipped, with trains on the surface and cars operating via subway, tweeting, “You can have hundreds of layers of tunnels but only one layer on surface (to first approximation), therefore trains should be on surface, cars below.”

Musk’s declaration attracted the attention of BART, which undermined Musk’s claim with stats about its own tunnel works. The transit agency tweeted, “We carry 28,000 people per hour through our Transbay Tube under the bay because of the capacity of a train. That’s nearly twice as much as cars over the bay. Why wouldn’t you prioritize something that carries far more [...] over cars?”

Here it is. We also run on electricity. pic.twitter.com/leK1mLzcrc — SFBART (@SFBART) May 25, 2019

Musk did not reply directly but continued touting his tunnel designs, noting that they could theoretically extend “from near your home [...] all the way to a destination parking lot.”

According to BART’s 2018 report updating the U.S. Congress on federal investments in local transit, about 14,200 persons cars cross the Bay Bridge at peak hours on weekdays, based on Caltrans figures on hourly auto crossings and the average number of riders (1.7) per vehicle.

The congressional report estimated BART could transport 27,000 people beneath the bay at the same time—slightly less than the number presented on Twitter, but still close to double the bridge capacity. Right now BART can run a maximum of 23 trains per hour through the tube.

Of course, the bridge is a bottleneck that naturally slows car traffic, which, more or less, makes it an isolated environment where BART can compare street traffic to train traffic. But it also means that the bridge is not a fair example of a typical Bay Area street.

Still, train travel is generally considered more efficient than auto transit.

According to the U.S. Department of Energy, “intercity rail,” “transit rail,” and “commuter rail” beat out the average car in per-passenger fuel economy, noting specifically that “all forms of rail achieve relatively high values due to high ridership and energy efficiency of rail transport,” although this varies depending on how close to capacity trains run.

According to Tesla, one of Musk’s Model Xs weighs 5,531 pounds, can reach speeds of about 130 miles per hour (although no road would permit this), and can seat up to seven.

BART says that a single one of its train cars weighs up to 20,000 pounds, tops out at 70 miles per hour, and seats between 50 and 60 people depending on the design—the new “fleet of the future” cars are at the low end with 50, ”but can carry over 200 customers in a crush load.”