I've had this problem myself in designing my layouts and I worked out a few simple rules, by experimenting and also by looking at others' maps that worked smoothly. Perhaps the best example is the "Wadalbavale to Karrah Bay" map in TRS2004.
The rules are these:
1.
If it's not visible from the in-cab cameras, it doesn't exist.
2. Every new different object I place costs ten bucks from my wallet.
3.
Every new instance of an existing object I place costs a dollar.
What this means is that you only place objects that are visible from the in-cab cameras on a loco when you drive past, and you use as few different object models as possible. I know from experience that the temptation is to build a complete town with houses, roads, streetlamps, stoplights, trees, bushes, fences and the whole box and dice, for miles from the tracks. DON'T DO THIS.
First and most important, don't go overboard using loads of different buildings. I generally restrict myself to no more than three or four different house models ($30-$40), two to three different shop models ($20 - $30), and two or three factory models ($20-$30), and repeat them over the whole town. The fewer
different objects you have, the less there is to load into memory. So a town with 150 houses all the same model will display much faster than a town with just 50 houses using 10 different house models. Do the same with trees - every house has the same tree type. Using this "cost model" will make you aware of how many different objects you're using, and make you minimise them effectively.
So, to design your town, place all your detailed stuff next to the tracks first. That includes your shops, factories, houses, along with road splines, yard fences, streetlights, garden flowers and trees etc - ONLY ALONG THE TRACKS, nowhere else. Now, zoom in as close as possible and follow the track with the right mouse button held down, as if you were driving a train. Look for gaps between your trackside buildings through which other more distant buildings would be visible, and note these gaps. Here, you can place extra buildings (of the same type you've placed next to the track to reduce memory usage) to fill in. Finally, any distant hillsides visible from the cab that have houses on them, have only low-poly houses used, no road splines or other detail at all. Use the grey "asphalt" textures creatively with a small brush to create the illusion of roads etc.
The art of map design - in any game, not just Trainz - is to create the
illusion of there being more than there actually is. Poly counts matter, so if you can get away with using a surface texture or a flat one-poly "billboard" object, do so. Keep solid high-detail objects for trackside only, and use only low-poly ones for further away. (In your cost model, flat "billboard" type objects are 25% of full price for a new object, so a new flat low-poly model only costs $2.50 to add, and 25c for each one you place.) Also, instead of covering a distant hillside with trees, use the "forest" textures on the landscape to create the illusion of trees. (This has the added advantage of making your distant hills seem bigger and more distant than they really are too! Turn up the fog a notch or two as well to enhance this illusion.)
Remember also that you see a LOT more in Surveyor than you do in Driver - in Driver, you're concentrating on driving the train, not floating about the map checking out the scenery. So be mean - really mean - with your object placement. Think about what a driver will see from a train, not what it looks like in Surveyor. Run-test your map LOTS during development - the "Quick Drive" (Ctrl-F2) feature was placed in Surveyor for a reason. It does take practice, time, patience and skill - but remember, it's all about "smoke and mirrors". If you follow these basic principles of minimalisation, it pays dividends when you come to play the game!
