Railway companies which operated the route from London Liverpool Street to Colchester (through Brentwood and Ingatestone) and Southend Victoria (from Shenfield) are listed below with links to Wikipedia.
1839. Eastern Counties Railway: http://en.wikipedia.org/wiki/Eastern_Counties_Railway
1862. Great Eastern Railway: http://en.wikipedia.org/wiki/Great_Eastern_Railway
1923. London and North Eastern Railway: http://en.wikipedia.org/wiki/London_and_North_Eastern_Railway
1948. Eastern Region of British Railways: http://en.wikipedia.org/wiki/Eastern_Region_of_British_Railways
(Network South-East was a passenger section created by British Rail, launched in 1986: http://en.wikipedia.org/wiki/Network_SouthEast)
1997. First Great Eastern: http://en.wikipedia.org/wiki/First_Great_Eastern
2004. National Express East Anglia: http://en.wikipedia.org/wiki/National_Express_East_Anglia