How many trips were made in total, and on what dates did they occur? What were the names of all the personnel involved, what were their specific tasks, and how did they become involved in the first place? Which day had the most trips, and which had the least? What kind of containers were used to hold the ash, where were they obtained, how large were they, and on average how many were placed on a vehicle? What types of vehicles were used, and how many of each type were used? How many trips made during the day and during the night? How much was spent on fuel? Which routes(s) was/were chosen to take the ash to its disposal, and who made those decisions? What ultimately was done with the ash containers and to the vehicles? Who were the persons in charge of this operation, and on what periods were they in charge? What sums of money were paid out to the personnel involved?
Quite a lot of interesting questions, and, according to forum rules, material for several new threads.
Let me, however, propose that you take a look at a map, which will answer some of your questions and make the rest irrelevant. You will see that the Vistula river surrounds Birkenau in the West and in the North. The distance from the places where the ash originated to the river banks is only a few hundred meters. Though behind the barbed wire (seen from the camp's interior), the right bank was within the Postenkette, as the river was a preferred site for attempting an evasion from the camp. Even if trucks would have been used for ash transport, they would have performed internal camp service. Transport would have needed neither "containers,"nor a lot of red tape. But let me remember that internal camp transport, as a rule, was done manually, by carts pulled by manpower.
