I have no idea. But I presume it might be for cosmetic reasons - so that the track ends wouldn't look so mismatched for tunnels where the track used is 'locked' and doesn't automatically change to whatever track type is connected to it. But that's just a presumption on my part.
As for road tunnels, I know jjslll54 made a modern looking Freeway tunnel for UTC - KUID:46819:37065, (if using the Black Pages, make sure 'UTC' ticked otherwise it won't appear in searches), but I'm not sure how compatible it is with the latest versions of Trainz.