And for the same energy, it cost 50% less.
The large differential between crude oil and natural gas commodity prices translates directly into a significant disparity between projected LNG and diesel fuel prices, even after accounting for natural gas liquefaction costs that exceed refining costs. In the AEO2014 Reference case, the long-run price difference between locomotive diesel fuel and LNG in rail applications increases from $1.48/gal of diesel equivalent in 2014 to $1.77 in 2040 (Figure IF3-2).
Yes, the railroad will need twice as big fuel tank. But what does that matter for a railroad? More importantly, for the same energy it has less mass.
And it warms up and loses density just sitting in an insulated tank.
Actually, when the engine is in use, the boil off of gas from the liquid provides an auto-refrigeration effect that cools the liquid. Changing from liquid to gas takes energy even without changing temperature. That energy is heat removed from the liquid. That is what makes LNG a better choice for engines that spend most of their time running and a poor choice for engines (like a personal vehicle) that spends most of their time waiting to be used.