With club layouts how many cater for changing periods during a show to allow various members stock to run - say X hours of BR steam, X hours of blue diesels, X hours of grouping?
Individual or small group layouts would not normally need to do this as they usually depict one specific period.
To cope with period changes:-
Do you change anything other than the rolling stock, or do you change vehicles, buildings or what?
Do you choose periods around transitions, like BR 1948-50 so you can mix grouping and early BR, or BR 1960s steam/diesel transition?
How do you cope with smaller detail like station names, signals - semaphore or colour light - road signs?
Domestic buildings in many cases you can get away with terraces and pre WW2 semis, (is anybody likely to nit pick older style doors and windows instead of UPVC double glazing?). Can you arrange to have lift out/drop in sections with different period housing?
High Streets are probably the most likely to define a period in an urban setting, from later 1950s onwards many smaller shops gave way to supermarkets, filling stations became self service with large roofed areas.
Many cinemas and theatres closed, replaced with other businesses - supermarket or retaurants, or bars, or betting shop.
On the edge of towns Industrial estates proliferated
Hiding the edges of any drop in sections needs some thought, hedges, fences and walls can work for housing areas, walls and fences along the backs of shops, for a street of shops I would suggest changing the complete street, shops both sides, pavements & road - you could also have period vehicles fixed in place on the road.
I'm sure there must be many other ideas to cope with changing periods.
At Bournville, the period is dictated by the presence of the engine shed, which was closed in 1960 and demolished by 62. A number of semis close to the track at one end of the line covered by the layout also locks us to mid 1930s at the earliest, but other than that the railway was surrounded by turn of the century terraces and allotments that changed very little, and still stand in much the same condition today (save for modern windows and street furniture). When the railway itself exhibits the time constraints, it makes things that bit easier. No diesels really, other than the odd early Rat and Derby Lightweight (please Farish!) and Bournville was known for keeping old survivors like some of the last outside framed Kirtley 2Fs into BR replacing them with ancient Johnson 2Fs that lasted right up to closure. So even modelling the earliest period we can the layout should have an even earlier feel to it than it is actually covering.
The joys of not having a station setting the period ;)