After he alighted at Milton Keynes the owner realised it was still in the busy carriage.
The painting was wrapped in brown cardboard packaging and is believed to have been removed from the train somewhere between Milton Keynes and Crewe.
Painted in 1954, Blue Harbour is valued in the tens of thousands of pounds. Its owner had brought it to London that day to be valued.
Anyone with information about the painting is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 and quoting crime reference 2000009076.
Alternatively, call Crimestoppers anonymously on 0800 555 111.