Software to do Duplo train track layout?
I would recommend BlueBrick, which is a versatile LEGO layout editor by Alban Nanty.

Apperently it has Duplo parts too, so I believe it should meet your needs. It can actually do much more for your layout planning needs (including modular tables if your club has some, for example), so maybe you'll consider it overkill as well, but since it's specifically made for LEGO layout I think you should be hapy with it.

Disclaimer: I have never used it for Duplo myself - if you do, feel free to share your experience with us in a comment.