It may be geography-dependent, but here in NE ohio, there are plenty of fleet vehicles and busses running on LNG (Liquified Natural Gas). For obvious reasons, the local NG utility has a bunch.
The big problem, as I understand it is the L in LNG. NG is methane, and holding enough to provide any reasonable range requires that it be liquified, which requires high pressures to do so and a suitable pressure vessel to carry it around. Once that’s in place, the conversion is pretty trivial, except for the cost of satisfying the EPA trolls.
Liquifying NG also requires a fair amount of energy to run the compressors.
It’s definitely do-able if there was some external factor driving a large scale conversion, but there’s a good reason liquid transportation fuels dominate the market.
LNG is not pressurized, it is cooled down to -260°F.
The pressurized systems, often 3,000~3,600 psi are CNG, compressed natural gas. It has more volume requirement but does not require cryogenics.