Aside from what the OPC are looking for (and although I have passed many of their station and on-train role assessments as a candidate, I do not know exactly how I would have answered this in the past), this is what my training and experience tells me to do:
1. A fire comes under the category of "train accident", at least in my copy of the rule book. Therefore procedures to be followed are those taken in the event of a train accident. Note "procedures" here. The OPC are not looking for people who make it up on the spot! They want people who, presented with a dangerous situation, can move calmly to working "out of course" and assessing / reacting to the hazard.
2. In considering this as a train accident, it is imperative that the train affected is stopped in a suitable position. A station may or may not be a good place - this is debatable. You usually have the optimum evacuation route in the immediate area by stopping at a station, but woe betide if the fire spreads to station buildings. But enough of this. You are stopped and, as it happens, in a station. I would probably have a train stopped there if I could, unless the fire was particularly vicious.
3. Which lines are obstructed by the fire, now the train has stopped? To understand this, you have to know what the fire is. You have been alerted, in this case by a passenger. So check out where and what the fire is. Don't assume one way or another that there is or isn't a problem. Tell other crew where you're going, and look for it. OK, so it's genuine, and severe, and you need all lines blocked at your location (for example). Then do that! The quickest method nowadays where I work is the GSM-R red button. Push it. If the radio works, make an emergency call. Block the lines and, if applicable, get an emergency switch-off. If it doesn't, five on the bell (staff leaving train), get to the nearest place where you can access the track safely, and lay TCOCs. Consider arrangements for emergency protection and which options present enough time to ensure immediate passenger safety. If the driver or other competent person is available, get them to lay detonators as per the Rule Book.
But... In practical terms, for either of these options, you may well have been in a cab first. So I would also have pushed the emergency stop button (before the GSM-R, actually, if I could), then the engine stop button, the hazard lights button (hint: if you don't do this before laying TCOCs, do it afterwards), and the switch for displaying a red light forward (if possible). Actions would be in that order. Each of these options takes a very minimal time.
4. Now you have an emergency call and line block etc. sorted, or protection using TCOCs, you have provided the best possible environment for an emergency evacuation, which is to be carried out when the train environment is immediately unsafe for passengers to remain in. Unfortunately, many passengers might not actually realise that, so at this point you must get people's attention via the PA and other safe means. Inform them that they must leave the train due to an incident onboard. Repeat the message once. Then work your way from the closest safe position you can get to the fire to your position of safety. Move people as far as possible from the train. Use everything you've got to keep people away. Don't discourage people from carrying luggage (it'll waste your time), but don't let them go back for anything. Don't try to rescue anyone where you would be in danger. And use other competent persons' skills to the best of your ability.