नहीं कोई गम, रेलवे से हैं हम - Aman

Even I thought so too, after seeing a sorting hub at ASK. But here the box is small, just for letters (less in quantity). So for sending letters to sorting hub wouldn't there be a lot of letters?
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There were notices on either side too. Train was on the move, so couldn't go and see.
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think that it's for employees of SWR or some sort of exchange of letters/envelopes to and from UBL -SWR HQ. This I say because stations like Yedakumari which were totally cut-off, in 2008 I saw a lot of correspondence in envelopes in a plastic cover was given to the LP of MAQ-MYS-SBC train. The train didn't stop but slowed down to receive the cover. Probably that was to MYS - HQ for that section. Anyway, just a guess. Fans who can have a closer look at the notice can give more info on this.
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See the attached pic. I saw it for the first time. A 'suggestion and complaint box' slot on the side of SLR. Then I saw the same on MYS-TLGP train too. Pretty common I guess, but I noticed for the first time. Wonder if officials really read the suggestions and complaints :)