Hi [NAME].Dell.User,   Unfortunately, there isn't a Dell online purchase for the rail kits that you have requested, as an example you have provided. I can only provide you the DPN# of the rail kit for reference. You may want to contact Dell sales to check if they are for purchase. They, too, may provide you different part number which works for the system.   MD12XX/32XX and PS6110/e : MH6DJ MD1000 : U9426   Yes the rails works for both storage. I've tried looking around in the part searches, no, there aren't different rails for 2U and 4U. 
I want rails for PowerVault MD1000, MD1200/3200 & EqualLogic 6110e.  Dell's rail sizing chart says type B9 for both MD1200 & EQL Ps61xx, and has no identifier for MD1000. But this identifier does not help on the pages listing rails for sale. (For example, I couldn't figure what the "Rail Identifier" type for this listing is). Can someone at Dell direct me to the right place to purchase B9-type rails? Also, 6110e is 4U & MD12xx/32xx is 2U. It seems the same rail works for both (Aren't there different rails for 4U & 2U?).  
Rail Kits for PV MD1000/1200/3200 & Equallogic 6110e