Trains would be nice but are way too far from the vehicles that we list, and naming of these would require specific fields different from vehicles I think.
A separate site would be better for that.

I was wondering if we did trains, I guess not?
Considering the fanaticism of many train enthusiasts, there's a good probability that some other site has it covered.

Here is a complete list: http://www.imcdb.org/search.php

Forklifts are listed.
Not the two others.

What about Boats, Jet Skis, and Forklifts?

For some like Lawnmowers we added few which had a role and were identified, but indeed usually we do not include these.

Other that are not listed:
- go karts
- electric chairs for disabled people

The only things we DO NOT except are:

- Travel Trailers (Caravans)
- Bicycles
- Lawnmowers

(There maybe more, but these are most prominent)

Someone please add any I may have forgotten.

we do accept all of those types of vehicles.

I know this is the Internet Movie Car Database, but would it be acceptable to include motorcycles and construction vehicles such as Bulldozers, Diggers And Excavators?

They all have engines, wheels (or treads...), etc.