1. To make the whole train lockable (only placer or op available);
2. To make more locomotives (Such as making it to use the creosote or the fuel in Buildcraft);
   These are from Chinese forum.
   I also think you can make another type of chest cart which can only be opened by the placer or the op.

The next official locomotive that is planned for sometime in the future(tm) will be the diesel locomotive.
However until then the API can be used to make add-ons that add custom locomotives.