The fact is either will work, however many people will find it much easier to use acid core solder when soldering nickel silver rail to copper ties as the acid will help to clean and etch the surfaces making it easier to get the solder to flow, especially when using small diameter solder which is easier to get into tight spots.
Rosin core solder will work equally well, but it takes more skill, heat and patience to create a sound, good looking joint.
Generally speaking acid core is not recommended for soldering small parts, electrical wiring or electronic components as left over acid will corrode and eventually weaken the joints, reducing their electrical and mechanical integrity.
However in the case of trackwork, it is easy to solve this problem by simply washing the completed trackwork in warm soapy water. (Something that is hard to do with electrical wiring and circuits.) As long as the leftover acid and flux is removed, then there is no danger of corrosion, and most people will find soldering much easier.
If you are an experienced modeller, skilled with the soldering iron, then using rosin core solder will eliminate the need to clean the trackwork when you are done. But beyond that, there is no real difference.
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