interchangeability??

[MBTA3247]

London like NYC has 2 different loading gauges, the subsurface lines (District, Metropolitan, Hammersmith and City, Circle) which were originally build to steam railway standards (and actually ran with steam power originally) and the "tube" lines which are smaller as they were early deep tunnels. Then there is the Waterloo and City which is a short 2 station shuttle between 2 key stations that has its own unique equipment. So different lines built to different loading gauges seems to be pretty common in the mass transit field, especially systems that have been around a while and with pieces built by different companies.

Jon

Adding to the fun in London, there are a few places where Underground and Tube equipment share the same tracks, and "mind the gap" ends up referring to the vertical gap between the platform and the lower Tube equipment.

To my recollection there have never been any physical connections between what is now the Orange Line and either the Blue or Red lines

You are correct, as they never came close to each other except where the tunnels crossed over/under in the downtown area. When the streetcar network was still intact, though, work equipment was easily shuffled around to wherever it was needed, as all lines had at least one connection to the streetcar system.

[Arborwayfan]

There's a detailed description of a hypothetical trip through all the El's and subways in Boston, connecting via streetcar tracks, somewhere in a thread on this site. It's from a few years ago.

Santiago, Chile has 3 connected lines of rubber-tire trains (cars only 7 feet wide inside, too) and 3 connected lines of steel-wheel trains (cars about 8 feet wide). A new line is being build with platform doors, but I don't know if the cars will be any different because of that. Certainly doesn't approach Boston.

Shouldn't we count the Mattapan line as a 5th set of equipment: trolley poles/wire vs. pantographs/catenary wire, plus other reasons the types 7 and 8 can't run there? Before the Boeings it was still part of one compatible green line fleet, and it would be easier to make it part of the same fleet again than to do the same for any of the other lines, but for now it's a fifth line. If someone already said this, I'm sorry.

[bingdude]

Not technically a connection with the Orange line, but the old viaduct that carried the Green Line over Causeway Street in front of the old Garden had a short stretch of elevated platform on the east end. It was a remnant of the old Atlantic Avenue Elevated, which crossed the old Orange Line on a diamond over North Washington Street. If there had been a side connection at that diamond, then orange line trains could have been run over it onto the Atlantic Ave track and then pushed right onto the Green Line. I doubt this ever was done but it is possible the old Atlantic Ave El could have been run onto the green line track.

[MBTA3247]

The original intention was indeed to extend Atlantic Avenue El trains over the East Cambridge Viaduct, but the closest that came to happening was the single stub end platform at North Station.

[Arborwayfan]

I believe the Atlantic Ave El joined the main line El rather than crossing it on diamonds.