The Rail Pass is valid on all train services operated by JR (Japan Rail Railways)
It is not valid on other privately operated railways.
Examples of private railways:
Keisei Express service from Narita Airport to Tokyo
Nankai Express service from KIX airport to Namba (Osaka)
Odakyu Express service from Shinjuku(Tokyo) to Hakone
However, the Rail Pass can also be used for a JR service—which may be slower or less frequent.
There are a couple of exceptional cases where the Rail Pass cannot be used, even though JR operates the services:
“Nozomi” bullet trains on the Tokyo-Kyoto-Osaka (Tokaido) line
“Mizuho” bullet trains on the Kyushu Shinkansen line