The P4s come via UMB, the P5s come via LKO, at least that's what I presume. Usually 2-3 P5s are available at MB (they're seen doing odd jobs like hauling SRE Pass and shunting coaches at MB). Whenever one of them needs to be sent home, SRE-LKO Pass takes it to LKO and Shatabdi takes it thereafter to Delhi.