Tell me about the railways.
I’d look about the internet for inspiration, there’s a number of things you want to consider, realism vs cramming lots of track in, do you enjoy shunting, shuffling operations or just setting things going round and round. If the former look into Shunting Puzzles, these are very compact forms of layout which are rewarding if you’ve well maintained locos that move slowly with ease, and can be tacked onto a larger layout as well or used as standalone. There are a number of pretty good model rail forums with many pictures of different layouts worth perusing, some get a bit STWish, some are quieter but cosier. If using 80s Hornby stock, a form of which is still available under the brandname ‘RailRoad’ disregard the temptation to get into digital… you can always convert later, but its the equivelnt of going 11sp big cumulative cost, and for a small garage wall layout, not really necassary. Also don’t look at the newer bachmann stuff, the detail will blow your mind and you’ll end up spending loads, I speak from experience!
But it sounds like an end to end, with singletrack weaving nicely along some scenery with a few sidings at the end for shunting could be easily* doable.
*not as easily as you’d probably like to think, it would be a commitment. Also if unused for a long time a strip and clean would be advised to protect against burning the motors out, I’ve seen this happen to mates’ stock.