That being said, I might redo my calculations for how fast troops can get to Canada under the assumption that a second sled route is set up along the south bank of the St. Lawrence with the same capacity as the original one, coming into operation on Feb 1, and treat that as 'canonical'.
It might be easier to work out how many men need to go down the sledge route/railway to clear the backlog, and what kind of upgrades that would require. They seem to have only used one train per day to take troops from Riviere du Loup, a decision I presume was inspired by the fact that Mason and Slidell were being handed over:
‘Arrangements have been made by the military authorities with the Managers of the Grand Trunk Railway, to have in readiness a daily express train, at Riviere du Loup, for the purpose of forwarding the troops on their arrival there, to any part of Canada on their line. Future divisions will be daily progressing from one part to another from St John’s New Brunswick to Riviere du Loup, so that when one division leaves a station, another will arrive at it, and in like manner will all the troops required in Canada be brought overland from Halifax or St John, New Brunswick.’ (Quebec Mercury, 16 January 1862 p.2)
‘The first division of the 62nd Regiment, amounting to about one hundred and sixty men, rank and file, arrived at Riviere du Loup, yesterday morning, at about ten o’clock, from Halifax, and passed up on their way to Montreal, this morning, where they will receive orders for their future destination. A second division of the same regiment arrived at Riviere du Loup, this morning early. This division will leave Riviere du Loup for Montreal, at about three o’clock this afternoon, by an Express train.’ (Quebec Mercury, 16 January 1862 p.2)
‘A division of the fifth battery of the 7th brigade of Royal Artillery, consisting of about seventy-five men and four officers, arrived at Riviere du Loup this morning from Halifax, taking about twenty-three days. They will, probably, not leave their quarters at Riviere du Loup before tomorrow morning, when they will then leave in the regular morning train for Quebec, which may be looked for here at about five o’clock, PM.’ (Quebec Mercury, 23 January 1862 p.2)
‘The troops conveyed up from River du Loup, yesterday, by special train, were seven officers and one hundred and sixty one men of the rifle Brigade. To-day, 7 officers and 167 men of the Rifle Brigade and 2 officers of the 62nd Regiment, are on the way up, and would arrive at the Chaudiere, about three PM.’ (Quebec Mercury, 28 January 1862 p.2)
‘Grenadier Guards- the first detachment of the Guards is expected from Riviere Du Loup this evening, from Halifax overland. Troops passing westward- Yesterday’s train conveyed 8 officers 165 men, Rifle Brigade, Major General Lord Poulet [sic] and staff, Captains Gordon and Seymour. Today, 1 officer, 16 men, artillery for Quebec; 4 officers and 149 men, Military train, and 1 man, 62nd Regiment, for Montreal, are en route.’ (Quebec Mercury, 30 January 1862 p.2)
‘Troops passing westward- Today’s train left Riviere du Loup at nine o’clock, am, with six officers and one hundred and sixty eight men of the Grenadier Guards, all for Montreal. Yesterday’s uptrain from Riviere du Loup conveyed 11 officers 156 men of the Grenadier Guards and 7 men of the Rifle Brigade all bound for Montreal.’ (Quebec Mercury, 1 February 1862 p.2)
It seems that normally there was just the one train per day:
‘Grand Trunk Railway. On or after Monday, January 6, 1862. Trains will leave Point Levi… For River du Loup daily, at 10:00AM. Trains will arrive at Point Levi… from River du Loup daily, at 3:20 PM’ (Quebec Mercury, 23 January 1862 p.3)
I don't know a great deal about the route, for instance whether it's double or single tracked. As such, I'm not really in a position to say whether three trains per day (one normal, two military) are feasible- let alone more. However, it seems easier to get more out of the existing routes than to extend a new one, particularly when you consider that the railway is far quicker than sledges in the first place.
the other possible trouble spot is Ogdensburg, where the river is about half a mile wide - it's directly opposite Prescott, which in the defence scheme was expected to host a field division, and I expect this is where the gunboat flotilla would have to run past some batteries - or outshoot them, of course
Judging by the
Official Record, they were planning several forts which they now don't have time to install. If they emplace batteries, though, they're going to have to be big ones: even without ironclads, the British have got the best part of forty gunboats coming down the river to shoot back. And the Union is pretty short of artillery as things stand.