Dear Jeff,
As described by you that:
In a case like that, you would make a single PO for 1 tank car (order unit), with a conversion factor for the relationship between tank car and KG (the order price unit). At GR, you can define how many KG you actually received.
This is what my requirement I will be knowing the Quantity only after I receive, so help me out how to create PO for such case.
Thanks,
Kumar.