Architectural vehicles on an HO layout
Here is how to populate your layout with lots of vehicles for a fraction of the cost of "real" HO vehicles. On eBay, look for "Architectural Vehicles". This search should get you there. The secret: these were never intended for model railroads at all. They are intended for architectural models, where architects have to fill up parking lots and street scenes with lots of generic vehicles to show clients.
The two primary architectural scales are 1:100 (good for HO) and 1:200 (good for N).
Take the scale designations with a big grain of salt. Here is an alleged 1:100 vehicle. I put my scale ruler on it, which says it is 14 feet long in HO. Then I went outside and measured my actual Kia Optima, a mid-size vehicle: 14 feet 8 inches.
14 feet long
These are NOT highly detailed models. They consist of three pieces: a black chassis/wheelset (wheels are molded in place, and do not turn), a clear window insert, and a body. You can snap them apart easily if you wish to paint the body a different color. You can improve their looks by painting the headlights and taillights, and a license plate.
All train stations need taxis. I made some by painting some cars yellow, adding a bit of styrene to the roof for a TAXI sign, and then adding a black stripe with a mini brush.
Cheap, but slow
typical eBay listing
The bigger the lot, the cheaper they are! Look for the 25 or 50 packs. You can probably get 50 vehicles for less than $20. These are mailed direct from China. It takes about 3 weeks to arrive in the USA.