Based on the name for the train in New York L-train is a name pimps like to give themselves considering that the L-train has 24 stops. So being L-trained simply means having a train run on you by a pimp named L-train or having a twenty four person train run on you. Also there was a well known pimp in Akron Ohio who was well known for running L-trains . Coincidentally he called himself L-train.
After Cynthia was L-trained by Big Mac she needed a full vagioplasty.