In some railcars there are seats positioned back to back, leaving an inverted-V-shaped space that may be large enough for a suitcase.
If your suitcase is too large to lift overhead and store above your seat (as mine always is), the easiest thing to do is look for space on the racks often positioned not far from the door of the railcar. If I don't see space there or on a mid-car rack (not always present), I look for space between the seats. It's unusual to have a problem finding space for my bag, which is about 24". It was an issue twice on French trains in August 2019. It seemed that the entire country was on the move that month.