Enable Multiple NewGRF engine sets
When enabled, this feature allows multiple NewGRF vehicle sets to exist side-by-side. Without this feature, the maximum number of different vehicles is severely limited and vehicle sets of the same type will overwrite each others vehicles and functions. Note: using multiple vehicle sets side-by-side might result in very high or very low purchase and running costs for vehicles. Mixing wagons and trains from different train sets might result in graphical errors.
Possible Settings: On or Off
In Openttd.cfg config file: ([vehicle] dynamic_engines = <true|false>)
If you're changing the setting in an older scenario or save game (with newGRF developer on) enter into the terminal ("set vehicle.dynamic_engines = on")
Sometimes this setting is also (wrongly) called engine pool.
Dynamic engines was committed to trunk in r12924.