B737 FLT CREW WAS ISSUED AN EXPEDITED RWY XING CLRNC AT PHL RESULTING IN A GAR LIKELY CAUSED BY CTLR TRAINING.

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AT APPROX XA35 EST ACR X COMMENCED A SINGLE ENG TAXI FROM GATE. THE TAXI CLRNC WAS SUCH: 'ACR X TAXI TO RWY 27L VIA TXWYS H; E; HOLD SHORT RWY 27R.' I READ THE CLRNC BACK; THE CAPT UNDERSTOOD THE CLRNC AND COMMENCED THE TAXI. AT THE INTXN OF TXWYS E AND K THERE WAS AN ASPHALT (DIP) AREA LEADING UP TO THE HOLD SHORT LINE OF RWY 27R. THE CAPT ELECTED TO MAINTAIN A SLOW FORWARD MOTION IN ORDER TO KEEP THE ACFT'S MOMENTUM; IN THE EVENT THAT WE RECEIVED A XING CLRNC. THAT WAY THE ACFT WOULD NOT GET BOGGED DOWN ON THE ASPHALT PORTION OF THE TXWY. THE GND CTLR WAS APPARENTLY (NEW); HE/SHE WAS BEING SUPERVISED BY ANOTHER CTLR. A BUSINESS JET LANDED ON RWY 27R; APPROX 20 SECONDS PASSED; AND WE WERE ISSUED A XING CLRNC. THE CAPT APPLIED 50% N1 PWR; THE ACFT SLOWLY ACCELERATED; THEN DECELERATED ON THE ASPHALT. THE SUPERVISING GND CTLR ORDERED US TO EXPEDITE. THE CAPT ADDED PWR; THEN UPON NOTICING AN A320 ON A 2 MI FINAL; ELECTED TO ABRUPTLY DISCONTINUE THE XING. THE ACFT HAD JUST STARTED TO ACCELERATE AT THAT POINT; AND CAME TO A STOP WITH THE NOSE GEAR ABOUT 4 FT ACROSS THE HOLD BAR OF RWY 27R. THE A320 ACFT WAS ORDERED TO GO AROUND; AND WE WERE TOLD TO CONTINUE TO CROSS. FACTORS: SINGLE ENG TAXI; LACK OF TIMELY XING CLRNC; TXWY CONDITION AT TXWYS E; K; AND RWY 27R; PLT LACK AUTHORITATIVE ACTION TO EITHER CROSS OR HOLD SHORT OF THE RWY; INEXPERIENCE OF GND CTLR; AND SUPERVISION OF HIS/HER ACTIONS BY THE TRAINING CTLR.