I was not aware of this proposal until, in 2006, I read the late Jack Edward's book "In a Manor of History" (a history of Steepleton Manor in Winterbourne Steepleton which sadly now appears to be out of print). I contributed material to Jack's researches.
In many ways, it would have completely altered the South Winterbourne valley. The route from Dorchester would have been relatively sensible along the South Winterbourne valley but once beyond Winterbourne Abbas, the terrain becomes far more challenging. The 1840's saw numerous railway routes proposed with all areas wishing to be connected by this new form of transport.
The proposed route was surveyed by local land surveyor John Martin (1780-1863) of Evershot. He had surveyed many Dorset villages for the Tithe maps of the 1840's and in his mid sixties embarked on surveying routes for proposed railways. Having surveyed this route and other options, he actually recommended the route be built from Maiden Newton to Bridport. This was completed in November 1857. It was extended to West Bay in 1884 but it ceased to carry passengers in 1930 as it was never really economic. It was cut by Beeching in the 1960's and is now disused like many such routes.
There is a superb website on John Martin the surveyor of Evershot - link : https://johnmartinofevershot.org/the-coming-of-the-railways/
I have also included an article below from the local parish magazine (Sept 2018) by local historian Terry Hearing and a copy of the original rather long winded application to Parliament posted in the London Gazette in 1845. This application named numerous villages etc so would have sufficed for the final route that did get built by the Great Western Railway.
Above : A locomotive in Bridport Station c1860
From the London Gazette (Part 4) : The Notice of Application to Pariament dated 3rd November 1845 proposing the railway line from Dorchester through Winterbourne Steepleton and on to Bridport. It was usual to include numerous villages and towns in order to keep route options open !