Irecently received a couple of reviews where my readers had problems with the co-existence of cars and steam engines. This, I must confess, I found baffling. Historically this happened. Steam trains were in commercial service in the UK until the mid 1960s. Steam traction engines were a common sight on the roads and especially on farms until the 1940s.
Ichose to have this mix of technology in my alternative future for what I thought were sound reasons. I believe that liquid fuels for road transport would be in short supply. The sheer technical difficulty of finding and processing oil into petroleum products makes petrol an unlikely fuel. The lack of readily accessible domestic sources of crude, in a low-tech world robbed of a great deal of specialist knowledge, also weighs against it. Any road vehicle using liquid fuels would almost certainly have to be powered by a diesel engine running on vegetable oils. This would be a relatively expensive and sophisticated engine.
The alternative is a steam engine, running on wood. Wood would be plentiful, require little processing and could drive a comparatively crude, low pressure, steam engine quite successfully. For scheduled operations, such as trains, or predictable commercial activities, such as hauling goods, a steam driven vehicle would be perfectly adequate and an alternative to heavy horses. Steam powered trucks were common enough on British roads up until the 1930s and traction engines were used for farm work and road mending until much later.