Just to be clear ... The Framework Agreement for Station Staff states:
Resident staff will occasionally be required to work at other stations within their group of stations. They may also be requested, with their agreement, to work at other stations in any job for which they hold the relevant licences, provided the requirement or request to work at another station arises out of an occurrence which was not anticipated. (Schedule 2, clause 1.2)

This means that if you are rostered, management can only require you to work at another station on your group if the need to do so arises from an unanticipated occurrence.

An unanticipated occurrence may include, for example: detrainment or other emergency at the other station which needs extra staff to help; a member of staff going off sick at short notice.

It does not include: duties being uncovered because a post has been kept vacant when it could be filled; duties being left uncovered because management have refused to offer them out on overtime; duties being uncovered because the rostered staff are away on leave or on a training course.

Management can only request you to work at a station off your group due to an unanticipated occurrence, and even then, this can only be a request - you are free to refuse.

Should you work at another station, all additional travelling time must be during your working hours.

If you believe that management are breaching these rules, or are putting undue pressure on you, please contact your RMT rep straightaway.