The flock are grazing outside Dorking Station, wearing jackets promoting and the fact that passengers can save up to 43% on their fares by booking through the site.
The sheep can be viewed from trains on the Gatwick Airport to Reading line.
The ticketing website's publicity stunt ties in with its new television ad campaign, created by DLKW, which broke last night.
This was shot in New Zealand and features 40 sheep, who were taught to baa on command and perform other feats such as stepping out of a taxi and pretending to read a newspaper. It has been filmed without any CGI.
Iain Hildreth, marketing director at thetrainline.com, said: "Sheep are a common sight from train windows right across the country. So, for us, using sheep as 'billboards' felt like an interesting and attention-grabbing area to explore, especially since sheep are featured so prominently in our advertising."
Sheep are the flavour of the month for advertisers - Samsung recently scored a hit with a featuring sheep with lights on their backs moving into different formations, promoting the company's LED television. .
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