Ukraine must bring all munitions from Poland or Romania, by train. The same with repairs of equipment (that is most all in Poland). Ukraine (like most of Europe), uses almost all electric trains (far more efficient than diesel).

That is why Russia has been targeting Ukraine's electric grid. It won't be just HIMARS that are affected. ALL munitions and equipment travel across Ukraine mostly by train. Hitting the Ukrainian electric grid is the most efficient way to derail (pun intended) Ukraine's war logistics operations.