Re: A hypothetical question

According to this document, the C&TS uses 4-1/2 to 5 tons of coal to get from Chama to Antonito.
According to this site, a ton of coal creates 4,172 pounds of CO2 when it is burned.
That means a one-way trip has a carbon footprint of about 4.5 tons coal x 4172 pounds CO2 per ton coal = 18,774 pounds, or about 9.4 tons of CO2.
A double-header likely uses about twice this, but could also have more passengers, increasing the footprint from the cars.

According to page 2 of this document, an average car creates 411 grams of CO2 per mile (based on fuel economy of 21./6 MPG). If there are 600 passengers (just my guess - could be way off), riding 2.5 per car, that's 240 car trips. If they are all driving from Denver to Chama, that's a round trip of around 600 miles.
600 miles per car x 240 cars x .411 kilograms per mile = 57,600 kg or about 65 tons of CO2.

So, 9.4 tons from the train, 65 tons from the cars.

Unless I slipped a decimal point somewhere, which is certainly possible, the cars produce about 7 times as much CO2. This doesn't mean that coal-fired steam trains are low-impact. Consider that the CO2 released by the train per passenger mile is about 0.63 pounds while the CO2 released per passenger mile in the cars is about 0.36 pounds.

The actual carbon footprint may be higher since some people will be driving much farther, and many will be flying before driving, but the average MPG could also be better since this will be mostly highway driving.

This does not account for the total impact because it does not take into account carbon produced by mining, drilling, refining, and transporting fuels, or many other less significant impacts, but it does look like the driving produces more CO2.