Hi Elle, sorry for taking so long to get back to you.
I only checked the definition of 'Rail' in the online dictionaries (Collins, TheFreeDictionary, and Chambers!) and they all confirmed what I had thought the meaning to be, hence my post. The annoying thing is that, not for the first time, I didn't bother to check the Chambers app on my phone, I just assumed the online and app definitions would be the same. I should have realised that the phone app, which you have to buy, is going to be more detailed than a free version - otherwise why spend the money?
From now on I'll make sure to use the Chambers app as my primary reference source. That's the plan anyway - again! :)