Quote:
Originally Posted by Migrant_Coconut
Handy reference - bottom line, single bore is cheaper and less disruptive, but much harder to build a station around. So they like to stick with double bore unless there's engineering issues (e.g. Burnaby Mountain).
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This way oversimplifies a complex problem.
The BART uses long and wide trains, their system also has drivers on every train which limits maximum frequency. In short, all infrastructure is used with a very low intensity. The diagram used also shows a double stack station which is probably more expensive.
The Skytrain uses a tiny train with high frequency to get the same capacity out of smaller infrastructure, In general a side by side layout (as in Evergreen) is doable because the trains are small. You can fit 2 skytrains side by side in a much smaller tunnel than BART or most other systems. This is the primary selling point for LIM trains in most places.
The only bad thing about the side by side layout is that you can't implement a Spanish Solution style platform at a later date. The stations also need 2 complete sets of elevators and stairs, but the box layout is pretty simple. The BART example needs a multi level station, escape stairways, and a structural deck between train levels. The Evergreen/Side-by-Side layout just needs a firewall and doors for fire escapes to be built inside the tunnel.
Larger tunnels are actually much more flexible in terms of geological conditions that they can work in. You can fit more equipment as well as larger equipment in the TBM. That in general is a big win.
The general opinion I've heard of the Evergreen Tunnel's issues was that the maintenance on the TBM was assumed to be much easier than it was in actuality. Optimism wins jobs, but pessimism should drive contingency planning.
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. But it won’t be as dreaded as boring through Burnaby Mountain, or if the contractor is The Boring Company.
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