I am not sure if there is a drivable excavator within trainz, but if you want to dig out some dirt with a drivable excavator, there is a good "oldie" PC game called "Construction Destruction".
It should be freeware in the meantime, but I can't say for sure.
I bought it for my little cousin a few years ago and it was quite entertaining, lacking features with lots of potential for improvement, but sadly .. dumped.
Yes, I am aware that this is not trainz related, but still .. it may just be of interest .