I don't have experience of just being months with a TOC, though I have been TOC hopping since I started in the Railway, found it hard to get promoted internally but faster to apply and get the promotion externally at another TOC, I'm probably not alone in doing that but I bet some have had the opposite experience and stayed years and years and been promoted a few times at the same TOC!
In my mind it doesn't look good moving around too often in jobs as it makes you look unstable, just my opinion though! Ideally you want a recruitment professional or line manager from the Railway to give their view but that's not my realm unfortunately.