B737-800 CREW HAD RWY INCURSION ON RWY 28 AND TXWY U AT BWI.

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WE HAD BEEN CLRED BY BWI GND CTL TO TAXI TO RWY 28 VIA TXWY U. ON TXWY U BEHIND ANOTHER B737 (#2 HOLDING SHORT); WE WERE ASKED BY TWR IF WE WERE READY FOR A 'RELEASE.' WE SAID WE WERE; NOT UNDERSTANDING THEM TO MEAN READY FOR TKOF. TWR REQUESTED THE ACFT IN FRONT OF US TO MOVE ONTO THE HOLDING AREA OUT OF THE WAY AND CLRED US FOR TKOF. I INFORMED TWR THAT WE WERE NOT READY YET; HAVING TO START THE OTHER ENG. TWR THEN REQUESTED THAT WE HOLD SHORT OF RWY 28; WHICH WE ACKNOWLEDGED. THERE WAS SOME DISCUSSION IN THE COCKPIT AS TO WHERE TO HOLD SHORT. BTWN MAKING PA'S AND STARTING THE #2 ENG; I REMINDED THE CAPT TO HOLD SHORT; WHICH HE ACKNOWLEDGED. WHEN WE STOPPED; TWR ASKED IF WE WERE READY FOR TKOF IN A MANNER THAT LED US TO BELIEVE WE MAY HAVE CROSSED THE HOLD SHORT LINE. IT WAS MY BELIEF THAT WE MAY HAVE GONE TOO FAR. HOWEVER; THE CAPT DID NOT THINK WE HAD. WE WERE INSTRUCTED TO TAXI ACROSS RWY 28 AND TO TAXI ON TXWY J AND HOLD SHORT OF RWY 28 AND TELL TWR WHEN WE WERE READY FOR TKOF. AFTER REPLAYING THE EVENTS OVER LATER IN THE FLT; WE WERE STILL NOT CERTAIN IF WE CROSSED THE HOLD SHORT LINE. DUE TO PLT WORKLOAD (STARTING ENGS; MAKING PA'S; CHKLISTS) THERE WAS NOT AS MUCH 'DUAL LOOKOUT' IN THE HOLD SHORT PHASE. 'INSIDE' THE COCKPIT DUTIES DETRACTED FROM THE OUTSIDE SITUATIONAL AWARENESS. THERE WAS ALSO SOMEWHAT OF A RUSHED ATMOSPHERE BECAUSE OF THE MISUNDERSTANDING OF WHAT TWR MEANT BY ASKING IF WE WERE 'READY FOR A RELEASE.' US HAVING REPLIED IN THE AFFIRMATIVE AND NOT ACTUALLY BEING READY FOR TKOF; MAY HAVE CAUSED A BIT MORE RUSH TO THE WORKLOAD THAN NORMAL. SUPPLEMENTAL INFO FROM ACN 495901: WE MAY HAVE CROSSED THE 'HOLD SHORT LINE.' WE STOPPED ABOUT 1 PLANE LENGTH FROM EDGE OF RWY 28 TO AWAIT FURTHER INSTRUCTIONS. THE YELLOW LINES ON TXWYS AND RAMP AREAS ARE VERY DIFFICULT TO SEE AT NIGHT IN THE BEST OF CONDITIONS. IN THIS CASE; PATCHES OF THIN PACKED SNOW AND ICE AROUND; FURTHER DEGRADES THE VISIBILITY OF THE LINES.