When you hover over the icon showing your rail and riverine, it will tell you you are 1/3 or 2/3 or 3/3. It's not a chance, but a definitive statement by the game. IIRC, it also tells you how much you need to improve to reach the next level. Do note that if you use your rail/riverine to move troops, then some portion will not be available for supply movment.
So if you have 110 rail, and you move a 90 weight corps by rail, you will only have 20 points left. The game engine will note that 20 puts you in the 1/3 category (for example, it could also be the 0/3 category), and only pushes supply by rail for 1 of the 3 pulses (or none). The 1/3 could be values from 10-100 rail points, or 20-140. It will depend on the side, and how much stuff you have (the rail requirements go up as you have more on the game - not sure if it is units or supply requests, but as your troops get bigger, you need a bigger rail pool to satisfy the demands of supply movement).
Remember - The beatings will continue until morale improves.