Complex sentence: Caltrain plans to complete the project by 2020, after which it plans to use double-decker electric multiple unit Stadler Rail trainsets on the electrified route, and retain the older diesel locomotives for service south of Tamien and, potentially, on the Dumbarton Rail Corridor.
Split into simpler sentences: Caltrain plans to complete the project by 2020, after which it plans to use double-decker electric multiple unit Stadler Rail trainsets on the electrified route. Some of the diesel locomotives will be retained for service south of Tamien and, potentially, on the Dumbarton Rail Corridor.