Just to expand upon this a little bit, railroads that operate under the General Code of Operating Rules establish their own set of signal rules, subject to FRA approval. Since Grant Tower is on Union Pacific lines, UP's signal rules apply.

Here's the rule and the specific wording for this instance:

Rule: 9.2.13

Name: Restricting

Indication: Proceed at restricted speed, not exceeding prescribed speed through turnout.

Thus, a train would need to be operating at Restricted Speed beginning the moment the leading end passes the signal, and remain at Restricted Speed until the leading end passes a more favorable signal or reaches the end of the signaled block. Because you're operating at Restricted Speed, you must be able to stop within half the visual distance anywhere within the signaled block, and operate not exceeding 20 M.P.H.

Additionally, because this specific signal has three heads (counted "A", "B" and "C", from top to bottom) and the "B" head is the one that's flashing, you know that you will be lined through a switch or crossover (the "turnout" mentioned in the rule). No part of your train can exceed the maximum authorized speed for the switch or crossover while passing over it.

So for example, if the maximum authorized speed for the turnout was 10 M.P.H., you would be required be at Restricted Speed not exceeding 20 at the moment you passed the signal, then reduce to 10 while passing over the turnout. You can increase your speed back to 20 once your train is clear of the turnout.

Clear as mud?