[ardecila]

^ Good question! There's a lot of confusion and variability here because railroads are governed by FRA and transit lines by FTA. It's just a huge bureaucratic barrier that keeps "commuter rail" and "transit" as separate concepts when other countries see it is a continuum between the two. The upshot is that every shared-use corridor like this is a special negotiation between multiple Federal and local agencies, and often freight railroads, and it's not based on predictable rules but on a case-by-case analysis. If you're planning a new project like Interboro, you have to assume that some stakeholders may demand very strict safety regulations.

In the case of the Green Line extension, I believe the close proximity of LRT and heavy railroad tracks was allowed because MBTA owns both lines, so they can be maintained together, and because the heavy railroad tracks basically see ONLY commuter trains and not freight. If there is freight, then it's likely MBTA agreed to restrict freight traffic to 1am-4am when Green Line service is not running.

The situation they're trying to prevent is this:
-freight railroad operates/maintains a track that is <25' from a track that is operated/maintained by a transit agency for light rail or subway trains
-freight railroad neglects maintenance, which is par for the course
-freight train derails and creates a dangerous situation for the transit service. if there is a collision between an LRT/subway vehicle and a freight vehicle, the freight will win every time because it's so much heavier and more rigid.

In the case of Interboro, there is a very clear desire to maintain the freight tracks for freight service, and it's unlikely that a time restriction will work because the subway system is expected to run 24hrs/day and the freight line may also need to carry traffic at all times due to the Cross Harbor Tunnel plan. So a physical separation is necessary. MTA has determined that they can't separate the tracks horizontally, so in the case of the LRT/BRT alternatives they're proposing to stack the new transit line over the freight tracks.