Supply cut off?

[Bigwig]

Well, you can't completely cut off supply just by severing a rail line, but if you set the supply display to the mode where it shows you numbers, you can tell if there is a break by checking for unusually low numbers. Unfortunately, the supply display, like the possession display, is affected by the fog of war, so the only way to tell where the break is that I know of is to visually check for broken rail hexes.

Of course, if you're out of supply, there won't be any bubbles in the supply display at all (I think), but you can also check by bringing up the unit report - the second item in the first line says "supplied" or "unsupplied". But being totally unsupplied generally only happens when a.) you are surrounded, or b.) you do an amphibious or air drop without a friendly supply source around, so it tends to be pretty obvious.