When you right click the timetable, it opens a gui. So you can set up different timetables for separate fleets of trains, this gui starts empty, but you can press the "add" button to add a train out of every constructed train in the list. It will then show this as a row, which has the train name on the left, a non interactable item slot in the middle, an item slot on the right hand side. You then have access to place a schedule in the second slot. It copies the schedule in, as it doesn't take it off you, but still keeps the item in that slot. If there is a schedule in, it will take it and apply it to the train selected, similar to the “Auto Schedule” feature of train stations, as in it applies the schedule, but you can’t take it back from the driver afterwards.
There is a "Publish" button which rolls out the new schedules to all the trains selected.
The middle item slot has a little label, “Current Schedule” which can be hovered over for simple view or left clicked for the full schedule gui, where nothing can be changed, only viewed however. When the changes are rolled out, all trains that have had a timetable change will allow the user to collect the old schedule from the left-most item slot