What is the minimum train tunnel height clearance in OO gauge?

There is no real reason why the height clearance of tunnels and bridges on a model railway should conform to any other rules except for making sure that any trains running on the line can pass through without fowling the roof of the tunnel.

Of course, if you reduce clearance too much, the visual effect of the portal may well become an issue. But every modeller will accept a certain amount of compromise to make things fit, so aesthetically, the only question is how low is too low for you.

Having said that, if you were simply looking for a rough guide to help you scratch build your own portals, a curved OO gauge tunnel mouth that peaks at around 9 cm high in the middle of the arch will appear very satisfying for most types of OO gauge rolling stock, the exception being tracks with catenary and tracks where rail cranes are to operate, both of which will require additional clearance.

One area many modellers do make a mistake is when reducing the curve of their tunnel mouth from its apex. If you curve the shape of your tunnel mouth too rapidly, the edge of your train's roof has the potential to clip the roof of the tunnel, so as well as height, considering the shape of your portal is something that should be done at the planning stage.