Indian Railways issues two kinds of Season tickets - at least on WR. One is labelled M/E (Mail Express) and other is ORD (Ordinary). On paper, a passholder with a ORD labeled MST is not allowed on Mail/Express trains. He will have to ask for M/E pass. ORD passes are technically allowed only for Passengers/Shuttles, EMU's and DMU's. However, if you notice, the fare for both kinds of passes is the same!
Strange Indian Railway rules!