As originally designed, the rate codes are
75 = "approach"
120 = "approach medium"
180 = "clear"
There have been other rates added, depending on the purychase (the Key System, GRS, used a 240 rate to signify exiting the train control limits).

These are just the basic frequency codes. There are time codes, polar codes, count codes, and various reverse codes. Source: US&S Bulletin 147, July 1943. No doubt the more modern variations of this system use different or modified coding schemes. Cab signal track circuits must use some frequency AC, preferably not a harmonic of 60 cycle commercial power, because DC can't be picked up by the [induction] receiving coils used. The PRR and NH (and Key System) used 100 cylce AC for their track circuits.

For situations like switches and diamonds, there is sometimes a short section cable which carries the codes [track circuit] around discontinuities. There is also a time delay of about 2-3 seconds between the change of a code and when it's displayed. This is for filtering out stray signals and those occasions when the code signal is briefly lost. Without this delay, there would be a lot of very unnecessary penalty applications, which can cause a long freight train to derail.