गाडी बुला रही है, सीटी बजा रही है

It is one of the unique characteristics of Bollywood lyricists to derive pragmatic values of life by observing everyday things — in this case a train. For example:

"responsibility of carrying people around, fire in its heart, smoke on its lips ... still keeps on singing, i.e., moving"
"[the train] won't bother about all that [thunder, rain & lightning], moves at the drop of signal, day or night"

Add to it the melodic singing Kishore Kumar can provide. A treat, although a nostalgic one.