First do NOT use it on your trains. It is not good for them. It contains a mixture of oils and additives that are not well controlled. There are significant chemical differences batch to batch. Some batches will attack the plastic in your trains. There can also be particles in it that will damage the bearings. 3:1 oil is a penetrating oil intended for old bolts; it is even no longer considered appropriate for lubricating locks.
Stick with high quality lubricants designed for hobby use, making sure that the particular grade you get is plastic compatible. Here are some of the brands you might find at your local hobby shop (LHS):
Atlas, Bachmann, Crest, Faller, Kadee, LaBelle, Life Like, Lionel, Woodland Scenics.
It does not matter which you pick, just stay with that brand.
3:1 oil is a “traditional” lubricant, but times and materials have changed, and it is not the best solution. There many who will say that they have used it for years, that is fine, but the materials used in model trains have changed greatly in the past decade.
I am a Mechanical Design Engineer, licensed Professional Engineer, with over 20 years experience. I design bearing systems for a living.