I used to be a revenue inspector for Merseytravel and a Special. I never mixed the two together and in 5 years only identified myself as a police officer once at a car crash. There's a couple of reasons for this... In the police I carried a radio with several hundred people behind it and available to help me in a matter of minutes. I wore an instantly recognisable uniform, had equipment and powers that generally everyone knows about.
As a revenue inspector, I had a phone, a uniform and powers that generally most people don't know about, but can guess (chucking people off, charging extra etc). I always carried my warrant card but that was more about knowing where it was than flashing it about. If I ever had any situations requiring police powers, I called the police. I would then identify myself to the police so that they knew I could do a statement etc if required.
In the case above the guard requests assistance by whatever way is appropriate. They may attempt to detain the person but even that is fraught with danger as they wont be wearing body armour, have handcuffs etc. And, as stated, as there has been a crime this could be a 'citizens arrest' - I found on a few occasions that other passengers will often help as well.
My advice to anyone joining was always to only identify yourself in these sorts of situations if you have already phoned 999 as any escalation of the situation may mean that you are unable to ring them after you have done it. These days too many people carry knives, screwdrivers, needles etc to take that chance.
