I really wonder what's the difference between one way signal and one way Path signal. I know they're not the same but my trains work with both of them. Also, building one way signal is slightly harder than building the path one.
One-way signals reserve a signal block between it and the next signal.
One-way path signals reserve an individual path between it and the next signal. This allows 2 trains to be on the same signal block at the same time, so long as they have paths that don't cross.
From the wiki:
For straight track and basic merges/splits, I stick to normal one-way signals. I use path signals for forks that have 4 or more tracks, as this is where they start to benefit.
Path signals also can behave similar to entry/exit signals, although entry/exit signals have uses that path can't fully achieve. For instance, entry/exit signals will cause trains to take longer routes if a path is blocked, while path signals may wait. This can be used to construct balancers and other helpful constructs.