So my grandpa was an engineer and I love railways... "different signaling systems, different voltages for electrical trains" isn't that much of a problem nowadays. There are almost always enough multi-system engines available that can cross borders and cope with those changes. We have a jumble of voltages in Czechia and yet another voltage on our German/Austrian borders, and trains go through without a hitch, not eve…
To deter further invasions, they could bomb/destroy entry points from Russia, Belarus and Crimea/Donbass as well. It would be quite effective, but deter commerce with the separatist regions. The Poroshenko government bought coal from Donbass after 2014, practically sponsoring the separatists.