  We report on the 2017 September 10 ground level enhancement (GLE) event
associated with a coronal mass ejection (CME) whose initial acceleration
(~9.1km s^-2) and initial speed (~4300 km/s) were among the highest observed in
the SOHO era. The GLE event was of low intensity (~4.4% above background) and
softer-than-average fluence spectrum. We suggest that poor connectivity
(longitudinal and latitudinal) of the source to Earth compounded by the weaker
ambient magnetic field contributed to these GLE properties. Events with similar
high initial speed either lacked GLE association or had softer fluence spectra.
The shock-formation height inferred from the metric type II burst was ~1.4 Rs,
consistent with other GLE events. The shock height at solar particle release
(SPR) was ~4.4+/-0.38 Rs, consistent with the parabolic relationship between
the shock height at SPR and source longitude. At SPR, the eastern flank of the
shock was observed in EUV projected on the disk near the longitudes
magnetically connected to Earth: W60 to W45.
