I have long fancied making a child friendly version of the Swiss Loesch und RettungsZug (Fire and Rescue Trains). This makes a nice brand jump between classic Town Fire & Rescue models, and the recently popular Train series.
The real life versions of these normally have a Tanker Wagon (with pumps), An Equipment Wagon (fire breathing apparatus, ladders etc) and a Rescue Wagon (for first aid). The first picture shows the Tanker and Equipment wagons. These normally ride together as a pair, and one half is traditionally motorized.
The Rescue Wagon could also be self powered, but would suffer from trying to fit inside battery, IR receiver and trucks. One alternative would be to supply a red locomotive (e.g. from set 3677)