It’s essential that you spend time to ensure:
– A believable back story if you are inventing a trackplan, or a believable version of a prototype as you will certainly have to squeeze a few dimensions – the real thing would be big even in 4mm scale.
– A plan that is operable in that it has sensible radius curves (12″ radius max if at all possible, 15″ if you can), few if any reverse curves without straights in between of a coach length, transitioned curves, sensible gradients, no cant and a decent overlap to ensure trains passing in loops cannot foul each other etc.
– A plan which will allow you to have the entire layout up, or significant parts of it allowing work to progress in all weathers and to enjoy operating it even before it is finished.
– An elegant look to the layout, less is more, curves are sexier than straights!