[HChampagne]

Why liquified? Compressed NG works well and there is plenty of room for storage even on an adjacent car.

[thackney]

Because CNG requires not only more space, but heavier cars to contain the pressure. More mass required to be moved to provide the same fuel.

[thackney]

More info at:

Natural gas fuels:
CNG and LNG
http://www.agilityfuelsystems.com/why-natural-gas/lng-vs-cng.html

[Freeport]

Volumetric (Vehicle range) reasons. Compressed natural gas is still 1000 times the volume of it’s liquid counterpart.

[thackney]

Typically for freight locomotives, the preferred natural gas medium is liquid natural gas (LNG). Due to its density, five times more LNG can be stored in the same size container than compressed natural gas (CNG), saving valuable space and making refueling less frequent.

http://www.energyconversions.com/tender.htm