Pay protection/fixation from higher grade pay to lower grade pay in same scale

Dear Sir/Madam, It is brought to your kind notice that i had served in one central govt deptt as GP A officer & also had gotten one promotion .but,due to selection in Indian Railways i tendered my technical resignation from previous deptt. for joining in later deptt.Details are as follows: date of joining previous deptt: 30/09/2004 pay scale:15600-39100 & grade pay 5400/ date of promotion in previous deptt: 1/04/2010. pay scale:15600-39100 &grade pay 6600/ date of resignation: 19/08/2012 date of joining Indian railways: 21/08/2012 pay scale:15600-39100 grade pay(New deptt.): 5400. Note:i was on probation till 10/12/215(Indian railways) My basic salary was 21290 when i joined Indian railways & GP 6600/. in view of above please guide me which pay will i get in Indian railways & increments on pay fixation. Thanks. Vedpal singh