What pollutes more cars or trains?

Rocket and planes.

Answer:

As detailed in the appended link the emissions from every mode of transportation vary so much depending on load, distance driven etc. that a direct and general purpose comparison is difficult to arrive at.

In most cases even finding a basis fo comparison is difficult although gms of emission per passenger-kilometer or tonne-kkilometer is common.

In the case of cars the answer id further confused by the availability of hybrid, electric, and osil fuel engines and motors, each with their own emission characteristics.