Assuming SMART will run 2-car trains, they will have 8 axles in play. As noted, those are light axles, despite the weight of the cars relative to light rail. Plus, it does rain in the North Bay. However, I suspect there will be more than just the shunts triggering the crossings most of the time - aren't they supposed to be tied in with signaling and PTC?

Light rail practice generally allows for single-car trains (4 or, usually, 6 axles) in 55mph (full speed) territory with grade crossings ... because the crossings are tied into the signals. If there's a signaling- or power-related failure with (a) crossing(s), there's a prominent light and notice to Dispatch, and trains approaching those crossings have to slow to restricted speed until they are in the crossing and it's clear, unless somebody is there flagging. Same thing if the driver observes the crossing to be partly obstructed, even if the track itself is clear (somebody's bumper over the line, for instance; Stockton Blvd on SacRT light rail is notorious for partial fouling) - restricted speed until clear of the crossing. Drives the people waiting for the train crazy; that could get interesting if SMART has similar rules (they're probably about the same all over) and more than a trivial number of crossing gate issues, given attitudes in the North Bay and especially Marin.