A reader from the UK writes:
We are looking to build and run a small gage sit on train track around our 7 acre garden and woods,
I have tried to keep track of companies that made trains big enough to ride on, but they tend to be independent craftsmen who try it for a while and realize they're not selling enough to pay their expenses.
These people seem to be in business as of this writing: http://www.lawntracks.com/locomotivechassis.html
They have an eBay store here, too. http://stores.ebay.com/LAWNTRACKS
These people seem to have some similar products available.http://www.realtrains.com/Ride-On-Trains.html
7.5" gauge is popular among "live steamers" in the US. Apparently 7.25" gauge is more popular in Europe.
Shipping will be prohibitive, I suppose. . . .
I can't vouch for either of these companies, I'm afraid. The only companies I really knew well have gone out of business.
Hope this gives you some idea where to start.
Best of luck,