According to A.J. Ludlams book "the Louth, Mablethorpe and Willoughby loop" (Locomotion paper No. 162 published by the Oakwood press, sadly now out of print). The last train from Mablethorpe ran on October 3rd 1970. It was the 7:55pm departure for Willoughby and Grimsby. The train was an overfilled 2 car DMU.
This picture shows a Brush type 2 (later class 31) on what would appear to be a summer excursion train arriving at Mablethorpe. All the short sleeves, summer dresses and shorts would bear out that fact. But none of that is important.
What is important are the people on the top of the footbridge. As my profile says, one of my earliest memories is of watching the trains from the footbridge at the railway station. So there is every chance one of those children on the top of the footbridge is me.