Well sport,
According to UP’s Instructions for Handling Hazardous Materials, Form 8620, (PB20800) effective 0900 CDT Monday, September 22, 2008…
Placement in train chart, page 40, column C, tank car, residue/empty shows no restrictions other than it may not be placed next to an engine, occupied caboose or business car…
If it is a unit train and all cars are carrying the same commodity or all cars are empty/ residue, the FRA says it may run with one buffer, they allow the un-manned locomotive to be used as that buffer.
Column D, loaded requires at least one buffer between a manned locomotive and between the load and the following, any class 1, class 2 or class 7, which are explosive, inhalation hazard or radioactive respectively.
Ethanol is UN 1170, it is a flammable liquid hazard class 3.
It may also be shipped under UN 1987alcohols, n.o.s., (not otherwise specified)
Empty/residue requires no buffer besides the un manned locomotive and has no restrictions on placement in train beyond the occupied caboose/business car restriction.
BNSF runs their loaded and empty diesel fuel train under the same conditions, using the un manned locomotive as the buffer.
Because diesel is a combustible class 3, the empty requires no buffer at all and can be used as buffer or cover car.