I'd say you have 3 options
1. Take the shunter job if the job and pay is better then what your currently doing, do your time in the shed and then apply for mainline internally (need to consider how many people will be ahead of you, also if you do go mainline c2c have this thing where you have to go part time minimum for 2 years before they consider you for full time).
2. Reject it and hold out for mainline somewhere
3. Take the shunter job but apply for mainline elsewhere. This is what I did. Some people in my course are still shunters or part time mainline.