A major issue I've run into with fluid wagon trains is that they need to leave a depot and have an empty inventory in order to load fluids into the wagon. The problem with that, is that in the vast majority of cases, the fluids being loaded onto the wagons will always be the same, so requiring it to both be stationed in a depot and have an empty inventory before being usable by the system is unnecessary in most situations, and simply adds to the time it takes to make a delivery.

A solution to this would be to add a "Use as Depot" option when the Cybernetic Combinator's mode is set to "Station", along with having "Provide only" enabled. Checking this would allow it to be used as a depot to the registered train, and would also disable the check if it has any cargo (an alternative check could instead be used to detect any contents in the train other than the type of item that the station is providing). This way, when a station requests fluids, there will already be a train ready to make a delivery.

An example: You could have a primary fluid providing depot. This would have a number of lanes feeding into it where other trains registered to this depot would wait. When the train that is at the depot receives a request, it leaves and is immediately replaced by one of the queuing trains to be loaded with fuel upon the next request.

I also noticed that trains are given the order to "Empty cargo inventory" when providing a request. On multi-fluid trains, this leads to the issue of them getting stuck if the requesting station doesn't need any of the other fluids that it's carrying. A solution to this would be to instead use the "Fluid Count" condition for the requested fluid, and setting it to "=0". For example, if my train is carrying Hydrogen, Oxygen, and Lubricant, and a station is requesting Lubricant, then the wait condition should be "Fluid Count: Lubricant = 0".