RWY INCURSION ACR Y NON ADHERENCE TO ATC INSTRUCTION TKOF WITHOUT CLRNC HAD NMAC WITH ACR X. ACR X EMER STOP BEYOND HOLD LINE.

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ACR X TAXIING FOR TKOF. WE WERE SEBOUND ON TXWY A (THE PARALLEL BTWN THE RWYS AND THE RAMP COMPLEX) WHEN GND CTL CLRED US TO CROSS RWY 30L AT TXWY H FOR TKOF ON RWY 30R AND TO 'EXPEDITE XING; TFC HOLDING IN POS AND TFC ON 2 1/2 MI FINAL.' TXWY H IS A TXWY WHICH CROSSES 30L APPROX 1700 FT FROM THE APCH END. TXWY H WAS THEN ABOUT 300 FT AHEAD OF OUR POS; AND IT WOULD BE A L 90 DEG TURN ONTO TXWY H. COPLT ACKNOWLEDGED THE CLRNC WITH A COMPLETE READBACK. CAPT REPEATED THE CLRNC AND ADDED A SMIDGEN OF PWR TO EXPEDITE. I OBSERVED ACR Y STARTING TO TURN INTO POS ON RWY 30L WITH LNDG/TAXI LIGHTS OFF. ANOTHER ACFT WAS A COUPLE MI OUT ON FINAL. IN ALL; THE SIT AND SPACING LOOKED COMPLETELY NORMAL FOR STL OPS DURING A DEP BANK. THE RADIOS WERE BUSY; BUT BY NO MEANS JAMMED. BEFORE WE GOT TO TXWY H; THE CTLR REPEATED OUR CLRNC TO CROSS AND TO EXPEDITE. ONLY A FEW SECONDS HAD ELAPSED SINCE HIS PRIOR CLRNC; AND HIS TONE INDICATED HE WAS CONCERNED ABOUT OUR SPACING AND WAS PUSHING THINGS A BIT. COPLT AGAIN ACKNOWLEDGED WITH FULL READBACK. CAPT REPEATED THE CLRNC AND DELAYED BRAKING THE ACFT AS WE APCHED THE TURN. JUST BEFORE WE GOT TO TXWY H AND THE CAPT BEGAN THE L TURN; COPLT CALLED OUT 'CLR ON THE R; Y HOLDING IN POS.' MOMENTS LATER; JUST AFTER WE BEGAN THE TURN; BOTH THE COPLT AND I OBSERVED Y ACCELERATING UNDER FULL PWR DOWN THE RWY; STILL WITH ALL LNDG/TAXI LIGHTS OFF. WE BOTH SHOUTED TO THE CAPT TO 'STOP THE ACFT; HE'S TAKING OFF;' OR WORDS TO THAT EFFECT. USING AGGRESSIVE BRAKING THE CAPT GOT OUR ACFT STOPPED WITH OUR NOSE GEAR JUST PAST THE HOLD SHORT LINES. OUR RADOME WAS ABOUT 125 FT FROM THE RWY CTRLINE AND STILL WELL CLR; ALTHOUGH LEGALLY SPEAKING WE WERE ON THE RWY. WHILE WE WERE STOPPING WE OBSERVED Y INITIATE AN ABORT; AND A COUPLE OF SECONDS LATER HE PASSED THROUGH OUR INTXN DOING ABOUT 80 KTS UNDER FULL REVERSE WITH SPOILERS DEPLOYED. OUR NOSE TO HIS WINGTIP CLRNC WAS 85 FT ASSUMING HE WAS ON CTRLINE. HAD WE NOT STOPPED HE WOULD HAVE STRUCK US ABOUT MID- FUSELAGE; IGNORING ANY LAST-DITCH EVASIVE MANEUVERS. THERE WAS TOTAL SILENCE ON THE RADIOS FOR ABOUT 45 SECONDS. WE 3 WERE STILL DIGESTING AND ACTIVELY DISCUSSING 'WHO'D DONE WHAT TO WHOM WHEN' WHEN THE NEXT ACFT ON FINAL; A BRASILIA; LANDED AND ROLLED BY OUR POS UNEVENTFULLY. THE SAME GND CTLR CAME ON AND IN HUSHED TONES CLRED US TO CROSS RWY 30L AND PROCEED TO RWY 30R. WE CROSSED RWY 30L. ONCE SAFELY ACROSS; THE CTLR CAME BACK ON AND APOLOGIZED; STATING THAT Y HAD BEEN 'CLRED INTO POS AND HOLD; BE READY FOR IMMEDIATE TKOF;' AND THAT Y HAD JUMPED THE GUN. THE CTLR TWICE STATED THAT WE HAD MADE NO ERROR IN OUR OPS; AND BY IMPLICATION; THAT NEITHER HAD HE. WE THEN SWITCHED TO RWY 30R TWR FREQ AND DEPARTED IN SEQUENCE NORMALLY. BEFORE WE TOOK OFF WE OBSERVED Y TAXIING BACK TO RWY 30L FOR ANOTHER TKOF ATTEMPT. AT NO TIME WERE WE ON THE SAME FREQ. AS Y I HAVE NO WAY TO VERIFY THE ACCURACY OF THE CTLR'S STATEMENTS. THE ACTUAL RADIO CONVERSATION BTWN TWR AND Y; THE TWR-TO- GND CTLR COORD PROCESS; AND THE Y CREW'S THINKING ARE NOT AVAILABLE TO ME. I DO NOT KNOW WHETHER Y ABORTED ON HIS OWN; OR IN RESPONSE TO DIRECTION FROM THE TWR CTLR.