If you believe in the global warming theory then yes. Burning hydrocarbons produces CO2. The more fuel you burn, the more CO2.
Prior to the global warming theory, the ideal was to burn fuel cleanly so that all of the exhaust was CO2 and H2O. Pollution was considered to be anything other than CO2 and H2O.
You can even download it in printable pdf format to show to any environmentalists you know. Just think of all the fun you can have undermining their doom mongering!
Environmental Effects of Increased Atmospheric Carbon Dioxide
Actually, the type of fuel used matters a lot. All trains run on electric power today. If they were using centrally generated power (via a trolley or third rail) instead of generating their own electric power on board with diesel generators, they would be substituting coal or nuclear sourced power for petroleum, which is the more limited resource in the marketplace.
The US (and England) have very large supplies of both coal and uranium on hand, if they can find the courage to use them.
No, there are many factors involved other than just fuel consumption. How much fuel and other raw materials did it take to make the transport in the first place? If a train requires 20 tons of steel and the equivalent number of cars required to replace the train weigh 40 tons, then you need to figure in the impact of getting all that steel. And, what about many roads required for cars, versus one train track? etc. etc.