In Voxel Tycoon, vehicles are the backbone of your company. You can't sell goods without them and as such they are essential. You can buy Road Vehicles in a Garage, whereas trains can be purchased in a Depot.
Each method has it's own strengths and weaknesses - Road Vehicles are cheap and are a good way to start, however they can't carry much and are relatively slow. Trains are much faster and can carry in bulk, however the infrastructure needed is a lot more expensive than that of Road Vehicles and needs a lot of space and forward planning.