Is it unrealistic to mix diesel and steam locomotives on the same layout?

It is not necessarily unrealistic to mix diesel and steam locomotives on the same layout. In fact, it was not uncommon for railways in the transition era (late 1950s to early 1960s) to have a mix of steam and diesel locomotives operating on the same lines.

However, it is important to consider the era and location of your model railway when deciding whether to mix steam and diesel locomotives. If your layout is set in an era when steam locomotives had been completely phased out and replaced by diesel locomotives, then it would be unrealistic to have both types of locomotives operating on the same lines.

Additionally, it is important to consider the purpose and function of the locomotives on your layout. If you are modelling a freight yard or industrial setting, it is more realistic to have a mix of steam and diesel locomotives, as different types of locomotives would have been used for different tasks. However, if you are modelling a passenger line in the diesel era, it may be more realistic to stick to diesel locomotives.

Overall, while it is not necessarily unrealistic to mix diesel and steam locomotives on the same layout, it is important to consider the era, location, and purpose of your layout. That said, like so many aspects of layout operation, the choice is an entirely subjective one and will inevitably reflect the personal inclination of the individual modeller.