49 CFR §213.143

1. (a)The guard check and guard face gages in frogs shall be within the following limits—
2. (b)For any heavy-point frog (HPF) on Class 5 track, the guard check gage may be less than 4′6 1/2″ but not be less than 4′6 3/8″, provided that:
   1. (1)Each HPF and guard rails on both rails through the turnout are equipped with at least three serviceable through-gage plates with elastic rail fasteners and guard rail braces that permit adjustment of the guard check gage without removing spikes or other fasteners from the crossties; and
   2. (2)Each HPF bears an identifying mark applied by either the track owner, railroad, or the frog manufacturer that identifies the frog as an HPF.