Other posters have covered Talltim's question adequately, but to add some more detail:
One vehicle is a HART vehicle. These carry specialist rescue equipment, including rope rescue, water rescue and CBRNE. The staff are also specially trained in rescue techniques and mass casualty events. Extracting someone from underneath a train is a technically very difficult task. I think there are 6 staff in a HART team.
One vehicle is a Paramedic Practitioner vehicle, who are senior clinicians with specialist skills and education in primary and urgent care. They are also used, as likely in this case, as a rapid response unit.
Three ambulances is a reasonable response for a report of two people being hit by a train, and due to skill mix, may mean there are four paramedics (with three technicians or care support workers [ECSW]) on scene to manage two major trauma patients.
In this area, one of the ambulances may have been a Critical Care Unit, staffed by a Critical Care Paramedic and an ECSW. These paramedics have masters level critical care teaching, and are specifically aimed at major trauma like this, amongst other things.
The helicopter will also have been dispatched to bring a doctor to the scene and provide rapid transport to a major trauma centre.
