>Would I be correct in thinking that "shunt" in this context is essentially a short circuit across the track, which pulls down the battery voltage allowing the relay to drop?

"Shunting" a track circuit is signal-ese for short circuiting it, which causes the track relay to drop, indicating "block occupied". For a train to shunt a track circuit, it's wheels must make good enough contact with the rails to conduct the track circuit current. Since it's voltage is rather low (typically between 1 and 10v) the wheel-rail contact must be good in order to conduct. Lighter weight rail equipment (Budd cars, these DMUs, etc.) are right on the cusp of being too light, particularly on a fully or partially corroded railhead. BARTD was having the corrosion problem when the Concord line opened with just the overnight fog and moisture; so was the BNSF in the late 1990s with nighttime crop fertilizing and weed spraying by neighboring farmers.