(https://img00.deviantart.net/2f51/i/2015/228/6/0/lswr_drummond_f9_inspection_saloon_by_rlkitterman-d95znjx.jpg)
Image linked from Google; Has anybody tried this in N gauge? How best might it be done? My thoughts would be a Japanese motor bogie under the saloon portion, hack loco body from a 14xx, saloon body cut from a Farish suburban. Needs 5'7" drivers, maybe adapt Farish 4P piston rod unless somethong better in the bits box.
It did get SR livery and was used in 1932 for a directors inspection of Southampton docks, so modellers licence would not be stretched too much.
I'd suggest maybe a Minitrix T3 chassis with the centre axle removed under the coach end. There's an old brass 14xx and auto coach kit that might be more bashable for the rest (I picked up a scrap one a while back. The loco being a dummy and the power being a Farish bogie motor of some sort in the carriage)
At least the carriage portion will be easy enough. I hope you make some progress with this.