UP's standard for anything with signals is a folded dipole on its own support mast next to the electronics dog house, which is usually right next to the signal mast. BNSF does the same thing.
In non-signaled territory, its placed next to whatever device, named point or when distance dicates an install.
For the NS comment above, not all "codeline" is RF, and its not always CTC. Codeline can be wired, wireless, fiber, sat or any other RF link.
When used with RF, its typically a form of ATCS, which there are several websites dedicated to all of that stuff.