Colour me curious. So based on the description as Loco Cleaning Oil / Smoke Fluid I assume this is a light kerosene similar to Marklin 02421 or 02423.

https://www.modellbahnunion.com/?ProdNr=SR24-1L

And hey, the bottle is very similar in design to 02421. Interesting. Anyway, I sure as hell wouldn't do this to my Flying Scotsman; but it could be an interesting approach to take for models with older open-frame motors. My biggest concern would be that there could be an chemical reaction with a component like the magnet wire sheathing or potting, that could cause a motor to fail down the line. At least on Marklin DCM or Tri-ang X03/X04 locomotives that's not a big deal as you can just drop a new 60941 and decoder; or have the Tri-ang commutator rewound. But I'd never do this on anything rare or hard to fix.

And this is basically just being lazy. There's a long-running thing in Leica camera worlds about technicians who strip a camera down to individual components, clean them, and reassemble them; vs those who dip the whole mechanism in solvent. Both improve performance, but the version of the process with more effort has longer-lasting gains. That said; where a mechasm is all of:

• Old/Simple.
  
• Cheap to repair or replace if it goes wrong.
  
• Difficult to disassemble without risking damage.
  

This is a good solution for a once-off hack.

My question is - what did they do with the solvent afterward? Did they manage to filter the crud out and re-use it? At €20/litre, it's not cheap!

And it appears that the lights (at least on one end... ) shall be nice and bright

Believe me, you don't want any liquid near such shutters. It will stick them together. The only option is to dismantle them and individually clean them and then assemble it once everything is thoroughly dry. The gear trains that set the shutter speeds need minimal oil - and I say minimal in the extreme - as otherwise the oil will work its way onto the shutter blades and once that happens nothing operates.