CL65 FLC TAXI ACROSS STL RWYS 30L&R WITH TFC ON FINAL. DISCUSSION WITH TWR DETERMINED ATC MISSED INCORRECT READBACK.

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WE HAD JUST COMPLETED A REVENUE FLT AND HAD UNLOADED OUR PAX AT THE GATE. CTL INSTRUCTED US TO TAXI VIA TXWY A TO TXWY N TO HOLD SHORT OF RWY 30L. WE DID SO; AND HELD SHORT OF RWY 30L FOR APPROX 2 MINS. IT WAS OBVIOUS THE GND CTLR FORGOT ABOUT US; SO WE REMINDED HIM THAT WE WERE HOLDING SHORT. THERE WAS 1 ACFT ON A LONG FINAL TO WHAT APPEARED TO US TO BE RWY 30L. THE CTLR CAME BACK WITH SOMETHING LIKE 'CROSS RWY 30L&R TO THE RAMP.' MY FO READ BACK 'ROGER; CROSS RWY 30L&R.' WE EXPEDITED ACROSS RWY 30L&R; UTILIZING OUR STANDARD CRM PROC AS WE APCHED EACH RWY HOLD LINE OF CONFIRMING WITH EACH OTHER THAT WE WERE 'CLRED TO CROSS AND CLR L AND R' WHILE VISUALLY SCANNING THE DEP/ARR PATHS. WHEN WE WERE 1/2 WAY ACROSS RWY 30R; THE GND CTLR (WHO I BELIEVE WAS ALSO WORKING THE TWR FREQ AT THAT LATE HR) QUERIED 'ACR X; ARE YOU HOLDING SHORT RWY 30R?' THEN CAME BACK WITH 'EXPEDITE ACROSS; THERE'S TFC ON SHORT FINAL TO RWY 30R.' WHEN WE WERE SAFELY ACROSS; I STATED THAT MY FO HAD READ BACK THE CLRNC TO CROSS BOTH RWYS. WE WERE TOLD TO CALL THE TWR; WHICH WE DID AFTER SECURING THE ACFT FOR THE NIGHT. WHEN I CALLED THE TWR; THE GND CTLR APOLOGIZED AND SAID THAT HE REALIZED THAT HE HAD FAILED TO CORRECT THE READBACK AFTER HE RAN DOWN AND LISTENED TO THE TAPES OF THE RADIO XMISSIONS. HE STATED THAT HIS ORIGINAL CLRNC WAS TO CROSS RWY 30L AND HOLD SHORT OF RWY 30R; AND THAT THE FO READ BACK TO CROSS BOTH RWYS; AND THAT HE NEVER CAUGHT THE ERROR. I DON'T BELIEVE THAT THERE WAS ANY SERIOUS LOSS OF SEPARATION IN THIS INCIDENT; AS THE LNDG TFC WAS STILL SEVERAL MI OUT ON FINAL; WHICH WE CONFIRMED BY DOING A VISUAL CHK BEFORE XING. FACTORS WHICH CONTRIBUTED TO THIS BREAKDOWN IN SAFETY INCLUDED THE TWR'S ORIGINAL UNCLR XMISSION; WHICH BOTH THE FO AND MYSELF MISUNDERSTOOD; AS WELL AS THE CTLR'S FAILURE TO GET THE STANDARD HOLD SHORT READBACK REQUIRED WHEN ASKED TO HOLD SHORT OF A RWY. I BELIEVE THAT A SAFER PROC WOULD INVOLVE REQUIRING VERBAL CONFIRMATION FOR XING EVERY RWY; ALTHOUGH THIS MAY NOT BE FEASIBLE AT A BUSY ARPT LIKE STL; WHERE FREQ CONGESTION ON THE RADIO IS AN ADDED PROB. PERHAPS AN ALTERNATE IDEA TO IMPROVE SAFETY ON THE GND WOULD BE A STOP/GO LIGHT SYS FOR CLRNC TO CROSS AT EVERY TXWY/ACTIVE RWY INTXN; ALLEVIATING THE NEED FOR RADIO COMS ALTOGETHER.