You can run dinky trolleys instead of substantial rail cars on the Beacon Line (ground-level access and safer to run next to bikers and walkers near the rail line), not interfere with federal adequacy laws or Hudson Line rail traffic, and still reach Beacon Station.
At first glance, to use the Beacon Line, to get to the platform at Beacon Station, you have to cross the busy main Hudson Line tracks. MetroNorth, Amtrak, CSX: by federal law their rail cars must conform to a minimum set of standards to use these tracks. For this reason, whatever rail cars you run on the Beacon Line must be substantial rail cars, not little dinky trolleys, since they would also use the main Hudson Line tracks.
Unless... you don't cross the main Hudson Line tracks at all.
At first, that doesn't make any sense, because you need to get to Beacon Station. Historically, there used to be a large web of tracks all over the waterfront in Beacon's industrial past. Wouldn't it be nice if one of those old lines still existed that doesn't cross the main Hudson Line tracks? Incredibly, one complete line does (and a few other fragments). It is buried in the weeds, but all of its rails are intact, all the way to the West parking lot of Beacon Station.
In this way, you can run dinky trolleys instead of substantial rail cars on the Beacon Line, not interfere with federal adequacy laws or Hudson Line rail traffic, and still reach Beacon Station.