How does that apply? It's an extension of an existing rail line.

The Boston green line is one of the oldest subways in the world and it shows.

The GLX, which is the part in question, is brand new. It definitely should not show.

it’ll show if the rail gauge matters for speed, which it does.

doesn’t matter how old the carriages are.

https://www.universalhub.com/2023/out-mouths-babes-green-lin...

I'm not sure I follow. Correct me if I'm wrong, but the Green Line (all of it, old and the Extension) use standard gauge tracks. The current problem with the Extension is that the brand new tracks are unstable and the parallel rails are collapsing into each other ("narrowing"). So it has nothing to do with the age of the rest of the system or the carriages, and everything to do with shoddy construction.

Yes, that’s my understanding, and my experience. The green line on the older tracks seems to be normal.

fair enough. what i was getting at was that even well made new construction inherits many constraints from the rest of the line, though it does ease others.

...but the actual oldest subway line in the world (between Paddington and Farringdon in London) works great. Recently got CBTC enabled so can handle 32tph (though doesn't actually run at that, though the not much newer southern half of the circle line does).

i bet you’ve upgraded your rail gauge geometry since original construction.

i’m much less confident that we have. or redone the tunnel layouts, hence all the squealing.

Yep, and the old ones in Hungary work well as well!