If a train is 100 km away, how much sooner would you hear the train coming by listening to the rails (iron) as opposed to listening for it standing by the track? in other words, how much faster will the sound travel through the iron rails than through the air? assume the air temperature is 0°c.

sound will reach you seconds sooner.

302

324

280

We . . . it's doubtful that you'd hear the sound of a train
from 100 km away (about 62 miles), but this is a fun problem
so let's go through it just for the math.

Online, I looked up the speed of sound in various materials.
Here's what I found:

-- Speed of sound in normal air . . . . .  343 m/s

-- Speed of sound in iron . . . . . 5,130 m/s 

But let's go a little farther !
Rails used to be made of cast iron or wrought iron. 
But now they're made of hot rolled steel.

-- Speed of sound in steel . . . . . 6,100 m/s

     Time to cover the distance = (distance) / (speed)

         Time through air  =  (100,000 m) / (343 m/s)

                                       =  291.5 sec  =  4 minutes 51.5 seconds

 Time through old iron rails  =  (100,000 m) / (5,130 m/s)

                                               =   19.5 seconds    (272 sec sooner)

 Time through new steel rails = (100,000 m) / (6,100 m/s)

                                                 =   16.4 seconds   (275 sec sooner)