To be honest I don't see the big deal. Armed police have been around on the railways for a long time now. I'm sure I wasn't the only one who was in London for the last official Gatwick Express 73 run, which came just 2 weeks after the London Bombings, and a week after the failed second attempts and on a day when there was a reasonable chance of something being tried again. London was swarming with armed police and police with sniffer dogs, yet there were loads of enthusiasts out and not one got bothered at all as far as I know. I remember kneeling down to take a photo of one of the locos at Gatwick and having an MP5 pass infront of me at eye level, the only issue being waiting until it and it's handler were out of the way so I could get my photo. After doing the trip I went to get in some slammers, and the same story was true in other places. The only trouble I had was one SET staff member at St Mary Cray who told me I couldn't photograph on the station, which didn't bother me as the only photo I took was off a VEP coming in that I was going to get on (the way he was going on I did worry he might try and remove me from the train, but I just got on and shut the door and window and he walked off.
The only thing I do worry about is the logistics of it, as BTP covers the entire country so therefore is either going to require to have large numbers of firearms trained officers (which will cost a lot of money) stationed all around the country, or will not be able to respond to incidents requiring an armed presence in most cases anyway. Basically they will probably just end up covering London, which is well supplied with armed officers from the Met anyway, and anything in the rest of the country will still require local firearms support just as now. BTP is best sticking at what they are good at, railway matters, and leaving more specialised operations, including those involving railways, to the local forces.