MLG FLC CLRED ONTO RWY WITH ACFT ON APCH. GAR PERFORMED.

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CREW GIVEN CLRNC TO MOVE UP AND HOLD SHORT OF ACTIVE RWY AFTER ANOTHER ACFT WON'T ACKNOWLEDGE SAME CLRNC. FO ACKNOWLEDGES CLRNC TO MOVE UP AND HOLD SHORT (1 ACFT IN POS FOR TKOF). CAPT TELLS FO TO MAKE PA ADVISING FLT ATTENDANTS TO BE SEATED. FO REACHES DOWN FOR PA AND MAKES BE SEATED ANNOUNCEMENT. AS FO FINISHES ANNOUNCEMENT AND REPLACES HANDSET NOTICES CAPT XING HOLD SHORT. FO STATES 'WE DON'T HAVE CLRNC.' CAPT SAYS 'I ROGERED A POS AND HOLD CALL' (FO HEARD NOTHING POSSIBLY DUE PA). ACFT ALREADY IN POS IS ROLLING. WE TAKE POS. ACFT ON FINAL ASKS 3 TIMES IF CLRED TO LAND DUE ACFT ON RWY. TWR REPEATEDLY ADVISES TFC ON THE ROLL 'CLRED TO LAND.' ON SHORT FINAL AIRBORNE ACFT AND TWR BOTH ACKNOWLEDGE THAT THERE IS AN ACFT ON RWY AND AN UNEVENTFUL GAR IS PERFORMED. DIFFERENT TWR VOICE MAKES GAR CALL AND STAYS ON FREQ TO RESEQUENCE TFC. SHORTLY THEREAFTER; NORMAL TKOF PERFORMED. NO QUESTIONS ASKED; NO STATEMENTS MADE; NO PHONE CALLS REQUESTED. DUE TO VERY HIGH TEMPO OPS; VERY BUSY 2-MAN COCKPIT; AND POSSIBLY CTLR TRAINING; SOMETHING BROKE DOWN. AT BUSY ARPTS RADIO COM ALONE FOR RWY CLRNC IS BREAKING DOWN. SOONER OR LATER LIGHT SIGNALS AT HOLD SHORT LINES MUST BE EMPLOYED. PLTS AND CTLRS ALIKE ARE ALL HUMAN. THE CURRENT SYS (RADIO COM) MUST HAVE REDUNDANCY; RADIO BLOCKING; AND THE INCREASING NUMBER OF ACFT FIGHTING FOR THEIR PLACE IN THE LINEUP MAKE FOR A VERY VULNERABLE SAFETY ENVIRONMENT.