Aurizon investors have been rattled by the potential collapse of Karara Mining as they assess the rail group's exposure to other iron ore miners.

Aurizon's shares tumbled 26¢, or 6.3 per cent, to close at $3.88 on Wednesday, extending a steep fall that began in late December after the company slashed its first-half profit forecasts. The stock is now trading at its lowest level since mid-2013.