Thinking about this in Simutrans-y terms...
All factories in Simutrans have a certain production rate, so an oil well produces oil at a certain rate. A pipeline has a certain innate capacity, since it's essentially a very long and thin storage tank. Until that pipeline has been filled, transfer of material from one end to the other cannot take place, but after it has been filled, transfer is virtually "instant", since one unit in displaces another unit from the other end.
Thus you could give each section of pipeline a storage capacity, say, 1 unit of liquid. A producing factory produces material at its maximum rate, and this then goes into filling up the pipeline's storage capacity. Once one section fills, the next one does and so on. If there are branches in the pipe then all the branches must fill simultaneously, and at each branch the fill rate will halve, since liquid must flow from one pipe into two.
If a factory has more than one pipeline removing material from it, then its production capacity is split between the pipelines, and if there's also a station to take material by train, then again the production is split between the three (in the same way as production is split currently if more than one player has a station connected to the same factory).
It would likely be a good idea to implement some ability to prioritize production for the various different forms of transport too.
Simiilarly, a factory which consumes the liquid (e.g. an oil power plant) has a set consumption rate, and would thus remove liquid from the pipeline at a constant rate. As it removes a unit from the pipeline that section of pipeline then becomes "empty", and can accept the next unit from the section of pipeline before it. There would be some factor determining how quickly this occurs, which would be a function of pipe diameter and material viscosity. Net result: a big pipe will transfer material more quickly than a small one.
This system means that you can have multiple "sources" and "sinks" in a pipeline network, and that the network itself has an innate storage capacity (which is realistic). Players would need to consider balancing the production and consumption in a pipeline network, or risk undersupply, just as they do with a rail or truck network.
You could have several different sizes of pumping station, at increasing cost, which would boost the flow rate through the pipe (and these could be powered to increase efficiency like factories). The basic pumping stations attached to factories to take away the material would have a certain base pumping capacity, which could be upgraded. As previously stated, a certain bore of pipe would have a set maximum capacity/hour for a certain viscosity of liquid, and the largest bore pipe wouldn't be completely fill-able by only one pumping station/factory, to encourage clustering of producers.
Pipelines IMO should take up a full tile, be buildable underground or over ground, be bridge-able (but only for 1-4 tiles or so, so they can go over roads etc, but not cross long stretches of water) - here tanker ships would be used.
Pumping stations would have to be built at each source location, which would cost more for higher capacity, additionally "receiving stations" would have to be built at the consumer, again with a cost based on capacity approach. All of this could be timelined too, so that better pumping technology and pipe becomes available over time.
That's just the way I'd do it though
Am kind of tempted to write a simulation to see if this would work.