While you're here, what do you think about adding a steam shovel? The change from pickaxes and shovels to excavators is enormous, but I think the progression of transport vehicles is very well done - every upgrade (minus powered cart ofc) is useful and reasonable.
A steam shovel would make dirt moving and roadbuilding easier, but if it's slow and clunky and can't dig over it's own height then it could have a good niche while still leaving a very good reason to upgrade to the excavator when it's available