These definitely wait until full before pulling away. They collect money whenever the mood strikes, and I've never been overcharged. The prices are really cheap. There aren't many of these, but if you want a colonial experience bar none, take one if you can. Better yet, go first class-what the heck, it's only a few dollars more, and with four rather than six people in your sleeping quarters, you reduce the chances of getting a loud snorer.
Some trains have dining cars serving meals.
If you need a dose of white linen, table service, and forks without spots, grab a bite here. But if you are on a quest for the perfect anecdote, sticking out your thumb may net you it. Many travellers in Southern Africa resort to hitchhiking. In many places, such as Namibia, there is little public transport, or almost none. So the travellers stick out their thumbs to get around cheaply.