Allowed speed is not as simple as just where you are; it also depends on the trackside signals and how the track in front of you is lined and/or occupied. For e

Allowed speed is not as simple as just where you are; it also depends on the trackside signals and how the track in front of you is lined and/or occupied. For example, if you are 1 mile from an interlocking you might be able to proceed at track speed because you will be going straight through it (clear signal) or you might need to slow down to, I.e., 20 mph as you are going to go through a switch to another track (approach diverging signal). One of the main complexities of PTC is getting all that signaling data onto the locomotive, combined with the route and rules, so that it can actually make the right decision. (Keep in mind that on the US's relatively slow freight railroads "cab signals"; I.e. seeing signal indications in the cab of the locomotive, is relatively uncommon. Signals are generally trackside only and are also often 50+ years old. They work fine, but they're not exactly networked in any modern sense. Updating all this trackside equipment is one of the primary costs of implementing PTC.)