Clywodd Trafnidiaeth Cymru am y gostyngiad ym mhris cyfranddaliadau Carillion fis Gorffennaf 2017, a thrafodwyd hyn gydag ARC.  Fe ystyriodd ARC yr opsiynau, gan benderfynu aros gyda Carillion, ei bartner o fewn y consortiwm.  O ganlyniad roedd angen gwarant ariannol annibynnol gan fanc ar Trafnidiaeth Cymru ar gyfer perfformiad Carillion.
Transport for Wales learned of Carillion’s share price drop in July 2017 and raised this with ARC. At this point ARC considered options but elected to stay with it’s consortium partner, Carillion. As a consequence Transport for Wales required independent financial guarantees by a bank for Carillion’s performance.