I think a Southern-based steam route would be good as there a few Sothern locomotives such as the S15 and Q1 but no Southern routes to run them on. I say that Waterloo to Weymouth and Bournemouth would give the train a long enough route to run on. The Main Line: Starting out of Waterloo and heading South-West towards Clapham Junction. At Pirbright Jn just after Brookwood, the train could carry on towards Basingstoke or the line could turn towards Alton and Arlesford (The Modern Day Mid-Hants Line) and travel towards Winchester and Eastleigh. If the train stays on towards Basingstoke, it could diverge at Basingstoke or Salisbury to make the run to Eastleigh. After passing through Eastleigh, the train could then run South West towards Southampton before heading West to Bournemouth or going via West Moors to avoid Bournemouth on the run to Weymouth via Wareham. Branch Lines: Swanage Branch: The Swanage Branch leaves the main line at Wareham heading South through towards Corfe Castle and Swanage. As the line is only 11 miles long, it is just the right sized branch to be accommodated in the route. As the line is only single line all the way into Swanage, there is a passing loop in Corfe Castle where there are 2 platforms instead of just one. Lymington Branch: The Lymington Branch is only 5.5 miles long with no passing loops along the whole line. The line diverges from the main line at Brockenhurst with only 1 stop in between at Ampress. After arriving at Lymington Town, the train then travels another 0.4 miles to Lymington Pier where the train meets the ferry for the Isle of Wight. Fawley Branch: Leaving the main line at Totton the train takes the 9.5 mile run running through Marchwood, Hythe and Fawley. The primary use of the line was for passengers but in 1943 a military port was built to help with D-Day landings in 1944. Locomotives: The Main Line locomotives on the route will be the Southern Railway's 4-6-2 8P Merchant Navy in British Railway Brunswick Green and Blue as well as Southern Malachite Green in unrebuilt and rebuilt conditions. Another main line locomotives are the West Country and Battle of Britain 4-6-2 7Ps. The liveries with these locomotives are Southern Malachite Green and British Railway Brunswick Green. More minor locomotives for local passenger services and freight services should include the Maunsell Q1, Schools and Lord Nelsons. These should have the BR Black and Brunswick Green as well as Southern. Branch lines could have smaller locomotives should be like the M7 Tank locos and the T9 Greyhounds. Rolling Stock: The main coaching stock for the main line should be the Bullied and Maunsell stock for passenger services as well as British Railway Stock. The Bournemouth Belle coaching stock including the brake, kitchen and even the observation car should be added. The stock for running freight services should be the generic British Railway stock as well as the Southern Railway box vans, both bogied and 4-wheeled as well as the 6-wheeled milk tankers. I hope this route gets created for TS2019 as the use of 64-bit would be good for this mileage and scenery intensive route. Thanks for reading
Luckily, part of the LSWR main line (including 'Castleman's Corkscrew') has already been done. A verfy well made steam-era route from Southampton to Weymouth is available from Steam Sounds Supreme: https://www.steamsoundssupreme.com/page_fwrp01.html. Also, Most of the engines and coaches you mention are also either in preparation or already availabe.
I'll second Marcus on that, it is a great freeware route. For an M7, a BR Standard 2-6-4 Tank, an Adams Radial and Southern PLVs etc then go to Digital Traction. For Greyhounds, Schools and N15 look here. https://steamlococollection.co.uk/ DTG/MatrixTrains Bulleid and Maunsell carriages are available in the DTG Marketplace.
