Looks like there has been a person hit at Chelmsford this afternoon, cancelling all trains east of Shenfield on that line. Seems that Southend branch is running fine.
My guess would be this was the service involved:
http://www.realtimetrains.co.uk/train/G01364/2017/04/12/advanced
My guess would be this was the service involved:
http://www.realtimetrains.co.uk/train/G01364/2017/04/12/advanced