If you're a TOC new entrant (meaning you entered employment with a Train Operating Company post-1997 - after privitisation), you get a PRIV which grants you 75% off Anytime tickets only (the most expensive type of ticket). Most TOCs also give you some form of free travel on their own services, and sometimes those of other TOCs owned by the same company - though there is no consistency between different companies, as the issusing of such a benefit is entirely down to the individual TOC. The same benefits the employee gets are usually granted to their spouse, and 'dependants' (children under 16, or 16+ but still in education/otherwise unemployed).
For example, all Govia staff (London Midland, Southern, Southeastern and Gatwick Express) get unlimited free travel on all services of all Govia TOCs. On Central Trains, we got unlimited travel on CT and a limited number of boxes for travel on other National Express TOCs (which were at the time Midland Mainline, Gatwick Express, one, c2c, and maybe another one...)
If you worked for the railways before 1997, it's all different, and I'm not entirely sure what you do and don't get.