I'm guessing it only runs on weekdays (since it's a peak extra)? Or could I pop up one Saturday and go for it or the 17:05?
No, absolutely Monday - Friday peaks only I'm afraid. And most definitely push pull.
The LFM trains belong to the genre of train that, back in the 'good old days' of the late 1970s, 1980s and 1990s, I used to call "the SNCF 5 o'clock". One or more loco hauled trains would depart at around 5 pm M-F on most lines out of every large French city or regional centre, with corresponding early morning inbound trains. They were easy to identify up to the 1980s when the paper timetables still showed 'autorail' at the top of the train column if the train was a unit. If the train departed at any time between 16.30 and 18.00 and was M-F only and had no 'Autorail' heading then one took an interest and more often than not it would be something interesting.
The only problem was choosing which line to do if there were several going off in different directions.
Here are some of my pictures of the genre from many moons ago...
First up, Nantes 67300 and DEV Inox stock
Limoges Benedictins. BB900 and 'Modernisés Est' aka 'Romilly' coaches
Roanne. 72000 and USI stock parked up after arriving from Lyon and running round to be set for the morning train back to Lyon (Lyon - Roanne had more than one of the peak hour extras as it is a very busy commuter route)
A real bonus. Double headed 72000 at Tarare (couldn't go any further towards Roanne that day as I had to get back to Geneva for the night)
even more hairy. An international version, Strasbourg - Offenburg, with scratch SNCF stock hauled by TWO DB Br181s!
Nancy. BB13000 with metalliques Est and 'Bruhat' stock