But it's nice to have things to look forward to, so here goes with another one.
Just as I wanted a 'pick up and put in a car by two people' battery electric I thought it would be good to be able to do the same one day with a live steamer. A coal fired one to boot. I don't think there will be a problem size wise but it's early days in the 'guess the weight of the loco' game.
We'll start with a mocked up picture with my cheap and cheerful Photoshop Elements software -

So, from left to right the thoughts of the Pell Wall Agricultural Tramway's chief engineer (I'm pretty sure I'm not worthy of capitals for that title).
Ah, should have mentioned, the base item is one of my flat waggons. The base height is probably much greater than anything that is ever built would be but the photo made for a useful kicking off point.
Cylinders.
I have shown twin oscillators as I believe that they may be cheaper to manufacture than more traditional types. Power to be transmitted, and geared down, via chains and cogs. Thus we avoid the extra complexity of quartering drivers, having coupling rods etc.
Brake.
Probably pretty much the same as for the battery electric loco
Seat.
Actually more than a seat as the space underneath would be for the water tank. Like the battery electric sideways facing .
Dropped footrest.
Although this was an initial idea for the battery electric it was not a requirement. It may not be here but I put it in to try and avoid forgetting the thought

Hand pump.
Those who know about these things say it is best to have two methods of getting water into a boiler (minimises the risk of things going BANG! and upsetting folk I believe). So a hand pump seems an obvious back up to either a steam injector or electric pump.
Boiler.
That boiler is about 10" high and 5" diameter, excluding the chimney and should be adequate. Again, with my cheapskate hat on vertical boilers are simpler and thus cheaper to manufacture than horizontal. It is shown sitting on a reddish box as this is envisaged to a. give a little extra height and b. one could fit in an ashpan to avoid hot coals falling on to plastic PnP sleepers. Not necessarily a good mix.
Note also the fire door faces sideways, not towards the driver. I think I am right in saying that Heywood locos are generally not fired 'on the move' and designing my loco like this avoids back breaking contortions. Run up and down the line. Pause to load/unload waggons. Feed the fire. Proceed. What's not to like?
Length: 40"
Wheelbase: somewhere between about 16" and 20"
Timescale: who knows

Any input, especially from those who actually do have engineering expertise, would be very welcome.
bestest,