Generally for the goods to be transported by rail, it has to be moved to the railway station using road transport. For that, an E-way bill would have been already generated and hence only the Part B needs to be updated with RR number for the rail transport. So an E-way bill for railways is complete and can be generated only after obtaining the RR number hence the concerned person/agent/supplier is responsible for getting this information.
For the goods transported through rail, Part B of the E-way bill can be updated either before or after the commencement of movement. But the railways will not deliver the goods unless the e-way bill as required is produced at the time of delivery. Indian Railways has been exempted from generation and carrying of an e-way bill but it is required to carry invoice or delivery challan etc.