First consider what the restraints have to do. Depending on the loco's proportions and the car's layout, miniature locomotives are often carried fore-and-aft. So the restraints stop the locomotive jumping as the vehicle goes over bumps; they hold it steady against lateral forces; and prevent it running or sliding lengthways. The last probably gives the largest loads on the restraints.
Each case is unique but common practice uses a stout base-board or frame with a pair of rails, to which the locomotive can be clipped or strapped down, with the whole lot itself restrained within the car.
You might, depending on the circumstances of use, also want to disguise the locomotive when it is in the car, for security. Some old blankets or the like are better than nothing, and I sometimes disguise the load as a heap of old plastic sheets, overalls and fertiliser bags!
My loco – a 7-1/4 " g version of the LBSC-design Juliet – is on square tube rails an open-framed steel trolley with large castors.
Anchoring the loco to the trolley are 4 hooked tensioners (a B&Q item), one on each corner. On each, one hook engages an eye on the trolley frame beyond the loco chassis; the other is hooked to a hole though the frame's lower corner. I forget if these holes were already there thanks to modifications, or if I drilled them specially. The tensioners can then be hand-tightened to secure the lot.
The important point is that the belays on the chassis are as far fore and aft, and outboard, of the loco as possible, to give low-angle tensions to prevent the loco from moving backwards and forwards, and sideways.
The trolley is secured to suitable points in the car – helped by that being a wheelchair-modified Renault Kangoo.
This assemblage managed the 120 miles from home to Warwickshire, and back, without trouble.
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In the loco's earlier career as a club portable-track work-horse, it was held down to a trailer floor or other suitable base, by two hardwood beams with semi-circular notches that engaged the buffer bodies; and each beam held in turn by a long central screw with a handwheel head, engaging a suitable bush on the base.
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Our club's Wren and petrol-hydraulic (both 7.25 " gauge) travel on rails bolted to a trailer floor but you could adapt the ideas for use in a car. The front coupling is shackled to an anchor on the trailer headboard, and the loco held down by ratchet straps with lots of padding between it and the locomotive. More padding around the engine gives more support and protection from anything rattling around them.
Many years ago we owned a 5" g. Maid of Kent. I think the tender was usually carried padded, on a spare seat in the car but the locomotive itself had its own box. This was wood, of inverted form, so the lid was the roof and vertical panels combined; with appropriate carrying-handles and sufficiently strong hold-downs to take the loco's weight. I forget how the engine was secured to the floor of the box, but it would have been by simple, padded straps or clamps of some sort.