before we could round them

Part of a story: (pardon my quoting long, because the tense might depend on a broad context)

"She is very fast," he said. "I doubt if we shall catch her."

"We must catch her!" cried Holmes between his teeth. "Heap it on, stokers! Make her do all she can! If we burn the boat we must have them!"

We were fairly after her now. The furnaces roared, and the powerful engines whizzed and clanked like a great metallic heart. Her sharp, steep prow cut through the still river-water and sent two rolling waves to right and to left of us. With every throb of the engines we sprang and quivered like a living thing. One great yellow lantern in our bows threw a long, flickering funnel of light in front of us. Right ahead a dark blur upon the water showed where the Aurora lay, and the swirl of white foam behind her spoke of the pace at which she was going. We flashed past barges, steamers, merchant-vessels, in and out, behind this one and round the other. Voices hailed us out of the darkness, but still the Aurora thundered on, and still we followed close upon her track.

"Pile it on, men, pile it on!" cried Holmes, looking down into the engine-room, while the fierce glow from below beat upon his eager, aquiline face. "Get every pound of steam you can."

"I think we gain a little," said Jones with his eyes on the Aurora.

"I am sure of it," said I. "We shall be up with her in a very few minutes."

At that moment, however, as our evil fate would have it, a tug with three barges in tow blundered in between us. It was only by putting our helm hard down that we avoided a collision, and before we could round them and recover our way the Aurora had gained a good two hundred yards. She was still, however, well in view, and the murky, uncertain twilight was settling into a clear, starlit night.
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In English, referring to a past particular situation to mean ability and using a positive sentence, we should use WAS/WERE ABLE TO DO, not COULD DO; but why does the writer use COULD? Isn't it a past particular situation?