I am not privy to the specific reasons for the speed limits in the Alameda Corridor, But as a signal engineer, I may be able to spread some light on the subject. First off, curvature is not the only consideration. Of vital importance is that the required distance to stop must not exceed signal spacing, or one half the sight distance without signals.

A train must be able to stop in the distance between the first yellow and the first red. In three aspect territory (Red/Yellow/Green) that would be one block length. If four aspects are used (Red/Yellow/fl-Yellow/Green), that would be two block lengths.

In either case, block length (signal spacing) vs braking distance dictates the maximum permissable speed that can be allowed in any given territory, irrespective of any further restrictions imposed by curvature. Steep downgrades may also limit allowable speeds. At a high enough speed, the pull of gravity will exceed the maximum braking capacity of the train, making it impossible to stop or even slow down.

The big problem here is that the required braking distance goes up by the square of the speed. It thus will take four times the braking distance for a train to stop from 80mph as it would from 40 mph. If you do the math (basic newtonian physics), it quickly can be discovered that running trains twice as fast will require blocks four times as long. It will therefore require twice the time to clear the blocks for the next train. Track capacity is thus SHARPLY REDUCED as you go faster ! Surprising isn't it!

Higher capacity will require shorter blocks, while higher speeds will require longer blocks. The two curves will cross at some point, which is the optimum speed of a given line. The exact optimum speed for maximizing track capacity depends upon a lot of factors such as train length, achievable brake rate, gradient, acceleration and such, which must all still be balanced with other economic and marketing considerations.

Considering track capacity alone, the optimum speeds for nimble passenger trains seems to be in the 30-45 mph range, and somewhat slower for heavy haul freight. This is the probable reasoning behind the design speed of the Alemeda Corridor. If someone here actually worked on the design, they may be able to be more precise.

Hope this helps

OPB