Firstly, there are 3 types of staff travel:
Residential - travelling to and from work.
Duty - travelling as part of your work whilst on duty
Leisure - off duty travel not related to work at all.
Generally speaking all discounted staff travel tickets are only valid for Leisure travel although you can buy a staff rate season ticket. This needs to be applied for.
In terms of travelling to or from work, this is an odd one.
TPE staff do get free residential travel on Northern services, it is printed on their staff travel cards.
The Old Northern used to issue about 25 different styles of staff travel cards, some of which weren't issued to staff but to all sorts of clingers on. Some were valid on TPE, some were not. The ones which were valid on TPE had it noted thereon. However generally the passes did not have a mention of TPE travel, which is somewhat odd seeing as TPE has it printed in theirs as being valid on Northern.
If you are in uniform it shouldn't be a problem travelling for free to and from work, like TPE do on Northern. Quite why Northern made their passes so complicated is quite beyond me.