ELI5 Train Tracks - how do trains maintain traction in slippery conditions?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

How do trains maintain traction on the tracks, especially during rainy or icy conditions? How do the wheels not slip off? Why do we always see stone or gravel on train tracks? How did early trains in the 1800s make sure they didn’t collide into each other?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: leprechaun2me, lblluke, kuddemuddel, toxiclayishimura_mjd, darthartero and dcsantonio

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