Born of frustration, here's a little Flash based tool I wrote that TS mappers such as myself should find useful. It allows you to draw tunnels (three pairs of them, to be precise) between two tunnel entrances and as an output you get the lines of code for the tunnels that you can then copy and paste into your map file. I've made several tunnels with this and so far they've all worked very solid! So in case you're struggling with curved or other more complex tunnels, this program is for you. :)