ACR Y DSCNT TO OCCUPIED ALT OVERTAKE HAD LTSS FROM ACR X. SYS ERROR.

Narrative

I WAS ADMINISTERING ON-THE-JOB TRAINING AT THE CARLTON RADAR POS. THE TFC WAS VERY BUSY AND COMPLEX. 2 SUCCESSIVE INBOUNDS TO DETROIT METRO ARPT WERE INVOLVED (ACR X AND ACR Y.) ACR Y OVERTOOK ACR X AT THE SAME ALT AND LOSS OF SEPARATION OCCURRED. ACR Y WAS CHANGED TO MY FREQ AT A LATER THAN USUAL TIME AND THIS ACFT WAS FASTER THAN I REALIZED. I ALLOWED THE TRAINEE TO WORK THIS TFC EVEN THOUGH IT WAS VERY BUSY. NOBODY; INCLUDING THE ASSOCIATE CTLR AND AN OBSERVING SUPVR NOTICED THE OVERTAKE UNTIL IT WAS TOO LATE. CORRECTIVE ACTIONS WERE TAKEN BUT IT DID NOT STOP THE ERROR. SUPPLEMENTAL INFO FROM ACN 194197: ACR X WAS BEING STEPPED DOWN ON TOP OF OTHER ACFT. ACR Y 15 MI NEVER CHKED IN; OR NEVER SWITCHED. ACR Y FINALLY TRACKED DOWN AND CALLED WAS DSND TO A CLEAN ALT; ACR X WAS BEING STEPPED DOWN ON TOP OF AND THE OVERTAKE WAS OVERLOOKED. THE TFC COMPLEXITY WAS EXTREME. ACR Y WAS TURNED OUT TO ALLEVIATE THE OVERTAKE; BUT TOO LATE. SUPPLEMENTAL INFO FROM ACN 194179: I WAS WORKING RADAR ASSOCIATE POS AT THE CRL SECTOR AT ZOB AT THE TIME OF OCCURRENCE. 2 PAX JETS INBOUND TO DETROIT METRO WERE DSNDING TO CROSS OUTER FIX WHEN THE FIRST ONE WAS GIVEN 120 WITH THE BACK ONE AT 160 AND OVERTAKING THE FIRST.