Getting a shot which is good for light of a down train in Cornwall around midday from a station is very difficult. Only decent shot for sun angle would be the road bridge by Marizion station
http://www.southwestrailphots.fotopic.net/p34108750.html, however Marazion station is closed so its a walk from Penzance which can be done, but its a brave distance (I've done it). Although there is a bus from PNZ station.
Although I expect Penzance is a little too far for you to come, only other place that may work is the bypass bridge a short walk from Lostwithiel station.
Then again what are the chances of it being sunny in Cornwall went something ineteresting happens??
N