SMART vehicles are not LRVs - they are modern equivalents of RDC cars, and afaik do have solid axles. Failure to shunt track circuits properly may be related to something else.

Older-style high-floor LRVs usually have solid axles, and generally shunt track circuits adequately. Low-floor LRVs and streetcars can have several configurations, including one where all the wheels are on stub axles; those may require special internal wiring to simulate solid axles for shunting purposes. In principle, they should work because the wheels are still completing the circuit through the car body for propulsion power and should therefore have electrical continuity between all the wheels; however, for low voltages like the signal circuits there may be too much resistance through all the body connections to allow enough current to flow.

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?