There should be but there currently isn’t one available and it’s anybody’s guess as to whether there will be one or when it might be available.
When I recently asked DTG support about this, they provided a link to the switcher's sales page, where a download link to the manual will supposedly appear -- eventually: https://store.steampowered.com/app/...d_Caltrain_MP15DC_Diesel_Switcher_Loco_AddOn/ .