Hi Guys,
The flux I use is Kester's SP-30 acid flux. The main reason I recommend this type of flux is that modelers new to soldering rail to PC board will find it much easier when using an aggressive acid based flux. The downside to the easy soldering is that the flux residue must be removed completely. Fortunately this is pretty easy to do using a good stiff brush.
When the soldering is complete I give the tops of the PC board a good scrubbing with one of these brushes until the copper foil is nice and shiny and all traces of flux are gone from the ties and the rail. This is good practice regardless of the type of flux being used as it will smooth out the solder joints and provide a good surface on the PC boards for paint to adhere to.
This goes quick and does a great job. Follow this up with a quick cleaning with soapy water, this will remove any grit left over on the turnout from construction.
Following these steps will ensure solder joints will stay strong for the long haul. I have turnouts over 15 years old built with acid flux and zero problems.
Painting the turnout will also create a barrier between any acid residue and oxygen, preventing corrosion.
That said, feel free to use any type flux desired, they will all work to some degree. Modelers new to soldering might find acid works well as it is quite aggressive when soldering rail to PC board ties, both of which are usually covered with oils and corrosion of some form, acid flux is aggressive enough to burn through all that.
I have been using the Supersafe gel flux lately and find it works quite well and is a good alternative to an acid flux if you are concerned about corrosion issues.