A B767 CREW; TAXIING FOR TKOF AT PARIS (LFPG) CROSSED A RWY THAT WAS CLOSED DUE TO CONSTRUCTION.

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RWY 26R AT CDG ARPT CLOSED FOR RESURFACING. RECEIVED CLRNC TO 'TAXI TO RWY 26L VIA TXWY T; TXWY W9;' WHICH WE ACKNOWLEDGED. TXWY W9 WAS A CORRIDOR ACROSS THE CLOSED RWY; WITH CONSTRUCTION ON BOTH SIDES. APCHING TXWY W9; WE ASKED IF WE SHOULD SWITCH TO TWR FREQ AND WERE TOLD TO REMAIN WITH GND. AN AIRBUS DIRECTLY IN FRONT OF US WAS HOLDING SHORT OF THE CLOSED RWY; WHICH PROMPTED A DISCUSSION IN OUR COCKPIT AS TO WHY HE WAS HOLDING SHORT OF AN OBVIOUSLY CLOSED AND UNUSABLE RWY. SHORTLY THEREAFTER; HE TAXIED ACROSS RWY 26R WITHOUT ANY CLRNC THAT WE OVERHEARD (POSSIBLE BECAUSE HE HAD ALREADY SWITCHED TO TWR WITHOUT OUR KNOWLEDGE) AND WE FOLLOWED HIM ACROSS. CDG GND INFORMED US AFTER XING THE CLOSED RWY THAT WE SHOULD HAVE WAITED FOR CLRNC FROM HIM. IT IS OUR UNDERSTANDING THAT THE INITIAL TAXI CLRNC; WHICH GAVE SPECIFIC ROUTING AND INCLUDED NO HOLD SHORT INSTRUCTIONS; ALLOWED US TO CROSS THE CLOSED RWY. HOWEVER; THERE WAS ENOUGH DOUBT AFTER BEING MILDLY CHASTISED BY GND CTL; THAT WE DECIDED TO ASAP THE EVENT. SUPPLEMENTAL INFO FROM ACN 556694: APCHING TXWY W9 ON TXWY T; WE ASKED GND IF WE SHOULD SWITCH TO TWR. GND SAID 'REMAIN WITH GND.' AN AIRBUS DIRECTLY IN FRONT OF US WAS HOLDING SHORT OF RWY 26R WHICH PROMPTED A DISCUSSION IN THE COCKPIT AS TO WHY HE WAS HOLDING SHORT. SHORTLY THEREAFTER; HE CROSSED WITHOUT ANY CLRNC WE HEARD (WE FOLLOWED HIM IN SEQUENCE). AS WE APCHED RWY 26R WE NOTICED A YELLOW SIGN SAYING 'DO NOT BLOCK RWY 26R.'