Former Transnet electrical engineer, Francis Callard, is expected to resume his testimony at the commission of inquiry into allegations of state capture on Thursday.
Transnet was forced to increase its foreign borrowings to fund advanced payments of R7.37bn for 1 064 locomotives, the inquiry heard.
According to Callard, who appeared before the commission for the second day on Monday, Transnet's foreign borrowing requirements increased from R15bn to R21bn in the 2014/15 financial year as a result of the advanced payments.
"The contract payment schedules negatively and unnecessarily impacted Transnet," he told the commission.