Railway Modelling

A compromise of P4 and OO is 'EM' which uses a gauge of 18.2 mm (0.717 in) with more generous tolerances than P4 for check clearances. It gives a better appearance than OO though pointwork is not as close to reality as P4. It suits many where time and improved appearance are important. There is a small following of finescale OO which uses the same 16.5mm gauge as OO, but with the finer scale wheels and smaller clearances as used with EM- it is essentially 'EM-minus-1.7mm.'
Couplers and connectors
In addition to different scales, there are also different types of couplers for connecting cars, which are not compatible with each other.