Having cajoled Colin Edmondson into designing and building the tiny i.c Scamp locos for 7 1/4" gauge the Stockers has been doing further pondering.
Scamp is a reasonably easy lift for a couple of bods. It is also small enough, at 845mm, or around 33", long to go into pretty much any car if you want to go out to play. If you're on your own undo a few bits and it can be broken down into three chunks any of which can easily be lifted by one bod.
Looking into the future, when I'm old and decrepit and can scarcely lift a pint glass (I'm currently 57, but would happily admit to a mental age of around eight) I have been wondering about still smaller, lighter stuff.
There's another component to the pondering...
Not everyone likes the noise or fumes from an i.c loco. You may worry that your play might not make the neighbour's day, even if it exactly the same sound as a motor mower (remember, the original Scamp is powered by a mower Suffolk Punch engine).
All of which suggests a battery electric loco.
Now, I don't know if the following is possible, but the 'ideal' would be something of modest power and speed (in my case it only needs to shuffle a few wagons about and then trundle across the garden with them with me on board) that can be lifted, probably sans battery) by one person.
So, with the magic of Photoshop I've reduced the already small Scamp by perhaps eight inches to form the first cut design of Imp, seen below on the right. It exists only as a set of electrons at present, but who knows...