Uh - because they get commission from SNCF? I presume. Same as maninseat61 gets a cut when booked thru his commercial site.

Explanation:

Captaintrain was bought by Thetrainline.com and renamed Trainline.eu

In April 2019 they introduced a booking fee, initially only for non-Europeans based on your IP address then (perhaps realising this was against EU law) they applied a fee to all bookings. I calculate the fee to be 3% of booking value.

They are now migrating Trainline.eu to Thetrainline.com, so UK tickets and European tickets can be sold on one site. If you type Trainline.eu into your browser it now redirects to Thetrainline.com.

However, for the time being the links to Trainline.eu on www.seat61.com are special affiliate links which go to Trainline.eu with no redirection and no booking fee. But this is temporary and when they correct a few problems with the migration (OBB integration wasn't working on Thetrainline.com last time I checked, for example), they'll ask me to change links and point to Thetrainline.com, with fee. I will then have to reword everything to make the fee clear. Loco2.com charges no fee, nor does oui.sncf though I have never got on very well with SNCF's own site.

3% on a several long trains bill could be sizable - a boost to www.oui.sncf.