Desert is loving hard because the cities and industries are far away and scarce. The hills aren't a problem, so no complaints there.
Desert is easy unless you're playing by yourself on a tiny map or something. Industries being far apart is a good opportunity to earn lots of money because cargo is worth more depending on how far away you move it. Probably the trickiest thing about desert cities is that they grow extremely slowly on their own. They'll only grow if they have water and food, then providing passengers, mail, and goods will make them grow a lot faster. If you find a city that's in an oasis (the ground is grass) then it will already have water and food and you don't have to supply it. This is a good thing since water isn't exactly a super valuable good to haul although it is necessary if you want to grow a desert town. I'll just assume you picked a desert town for this next paragraph though.
Pick a city with some water nearby. If it's close enough that you can haul it with trucks then do that, but chances are you're going to want to use a point to point train network with terminus setups (the little x in front of the station to switch trains around). Run 2 or 3 water pumps into some other towns. The amount of product you deliver to a town doesn't matter, giving them 10t of food or 100t of food will grow them the same. Running 2 or 3 water pumps will develop a steady income and give you a couple cities to start growing though. Now that you've got some money, start running stuff from farms and plantations into canning factories, then dump the food in the same town. Now you'll have enough money that you can start to do whatever you want. Build up a second city like the first, then start running passengers and mail between the two. I usually just stick a mail car or two at the end of my passenger trains. Passengers grow cities the fastest and as they grow you'll get more, so it's a very lucrative industry.
After a while you can start supplying rubber, copper, and wood to factories and goods to towns (although I think they'll grow fine with just water, food, and passengers). Once a bank is build you can start bringing in diamonds, although I've seen people go bankrupt trying to collect diamonds with helecopters so watch what you're doing. I usually ignore diamonds.
Now that you've got a couple of towns you're probably rolling in cash. Continue to build up your rail network and then stop once you get to 2050 or whatever year maglevs become available. Find a place with 2 or 3 oil wells in the ocean. Then raise the land around them and build a train station in the middle of the ocean (trust me). Go to the opposite end of the map and put another station at an oil refinery. The reason for this is that the amount you are paid for a delivery is based on the distance it travels and how long it takes. So if you run a straight maglev track across the map with huge oil trains you will end up with trains that literally make millions of dollars every time they unload. Don't forget to take the goods away from oil refineries and dump them in one of your main towns or the refinery will go out of business.
So on sub-tropical maps, here's what I generally use:
Water > Water towers in a town without water
Fruit + Maize > Food Processing Plant, Food > Town
Rubber + Copper + Wood > Factory, Goods > Town
Oil > Refinery, Goods > Town
Stuff I might haul:
Diamonds > Banks
90% of the vehicles I own are trains. They're by far the most cost effective means of hauling anything. I use road vehicles for short deliveries and as feeders for huge passenger networks. Same with planes - I generally use them to get passengers from outlying cities to the huge train station in the main city. Other than that I generally find planes useless. Boats are okay for some deliveries but it has to be a cargo that doesn't lose much value over time. For example, I don't use passenger boats except between neighboring cities but they're okay for bringing oil to a land station for transfer to a train.
