
Under the deal, KBH will expand its traincard ad panels across the entire South West Trains fleet. Ads will now be available in all 1,328 carriages, compared with the 354 carriages previously available.
KBH will launch the new panels on the rail franchise with ads for the Royal British Legion.
The contract was renegotiated without a pitch, though KBH is understood to have been working with South West Trains for 18 months on the expansion and upgrade of the offering.
Martin Kelway Bamber, commercial director of KBH, said the renegotiation represented the next phase in KBH's development and that further expansion at the company was due later in the year.
He said KBH was now attracting new clients who were "recognizing the exceptional value the medium provides" for targeting affluent rail audiences.
South West Trains operates routes out of London's Waterloo station across the South West of England, to destinations including Southampton, Bournemouth, Exeter and Reading.
KBH said it now had a total of 50,000 panels - reaching 2.5million passengers a day according to TGI - on its London commuter routes, which include First Capital Connect Thameslink and First Capital Connect Great Northern, First Great Western and Southeastern.