In response to my analysis of SMART’s potential for double-tracking, commenters Richard Mlynarik and the Drunk Engineer pointed out that other rail lines run high frequency with sidings only and not a full-fledged double track. SMART, they reasoned, would save a bucketload of cash by building something similar, and they're right. At 15 minute headways, SMART will have at most 6 trains going in each direction once it reaches full build-out. If they stick to precise scheduling, they will pass at 6 predetermined points. Under the current plan, SMART will run 30 minute headways under a similar scheme, with only 3 passing points of 4 miles each. At that 4-mile standard, we would need another 12 miles of track (another 3 sidings) to permit 15 minute frequency. While my original assumption was for 56.7 miles of construction (70.5 miles minus the 1.8 mile Puerto Suello segment minus 12 miles of passing track), with this dramatically reduced need for new tracks we can shrink the cost by a similar margin. Rather than cost $284 million, 12 miles of track will only cost $60 million. That's much more reasonable.
California regulations treat sidings differently than regular two-track systems, and pegs the minimum width of the right-of-way at 50 feet, rather than 44. While that means the sidings will interfere with the mixed-use path in the narrower segments of the right-of-way, moving the path is far cheaper than extraneous track.
Though this doesn’t give SMART operational flexibility to raise and lower frequencies or speeds at will, the currently planned system doesn’t either. Any changes in frequency or speed will require some capital investment to ensure passing tracks are where they need to be. Compared to the cost of SMART's trains, though, it's not much of an expense.