sentences:  In January 2008, Angel Trains, on behalf of London Overground and Chiltern Railways, ordered 12 two-car trains for delivery, which would have been delivered during 2009.But difficulties over deliveries by Bombardier's suppliers led to delays in filling the Class 377 and 378 "Electrostar" orders, with subsequent knock-on delays for building the Class 172 sets.Work began at the end of 2009 after the freeing-up of one of Bombardier's production lines following completion of the Class 377 order for First Capital Connect.