Yes Rio, using vehicles definitely is a good way to 'age' a layout.
When I realised that I would have to incorporate dismals onto the layout (because of the tortuous inclines and diesel locos having more traction), I had to have the option of bringing the era from the original 1920s-early 50's on into the early 1960's. So I obtained some late-50's and 60's vehicles to also pepper the scenery.
Since then I found ways of having steam locos pull trains up the inclines (multi-heading, motorising freight and passenger 'helper' cars, using geared locos etc) so steam can dominate, but diesels still have their day so the eras mix of vehicles appear especially when friends or neighbours came over or on open days.
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