The three staunch and magnificent steamers belonging to the company, the Plymouth Rock, Western World and Mississippi, owing to the hard times have been laid up at their dock since the fall of 1857, to the great regret of the public generally, as well as to the detriment of the business interest of our city. With the return of a more prosperous era they will doubtless be again placed in commission. The line formed by these boats is the most pleasant and expeditious medium of communication between the East and the West and Southwest, and cannot fail to be well patronized, especially now that the Dayton and Michigan Railroad is completed, which will bring a large amount of both freight and passenger traffic by way of Detroit that formerly sought other routes.

The rolling stock now on the road consists of ninety-eight engines, seventy first class passenger cars, twelve second class cars; twenty-nine baggage cars, and two thousand seven hundred and seventy-eight freight cars, making a total of two thousand eight hundred and eighty-nine cars and all of which were built in the company's own shops.

This road is one hundred and eighty-eight miles long, and has been in operation throughout its whole extent since November, 1858. It is deserving of the distinctive appellation of the Back Bone Road of Michigan, having been of incalculable value in developing the resources of the region through which it is located, decidedly one of the richest and most important in the West. The principal towns and cities upon its line are Pontiac, Fentonville, St. Johns, Ionia, Grand Rapids and Grand Haven. The growth of these places has received a great impetus since its completion, while numerous villages have also sprung into being as if by magic at various points along the line. These changes are plainly visible in the improved trade of our city, and the increase from the same cause, must continue to be strongly marked. Last season over one-fourth of the wheat and wool received here was by this new route, and a number of vessels loaded at the company's noble and spacious wharf for European ports direct.

Within the year past, the company have completed one of the finest railway wharves in the world. It is 1,500 feet long by 90 broad, the west end of which is occupied by the freight house, the dimensions of which are 450 by 132 feet.

One of the most important events to Detroit and the entire West, that has transpired for many years, is the completion of this great thoroughfare. The link from Port Huron to this city was opened to traffic on the 21st of November, since which date the businesses crowding upon it has fully equaled its capacity. It is the Minerva of railways, having reached at a single bound a condition of prosperity outrivaling many of the oldest established roads on the continent.

It possesses important advantages over any other road both for freight and passenger traffic. Being of uniform gauge, no change of cars will be necessary from Sarnia to Portland; and being also under the management of one corporation, it affords better facilities for the protection of passengers and the preservation of their baggage than where they are required to pass over lines under the control of different and perhaps conflicting corporations. Having only one set of officers quartered upon its exchequer, it can afford to do business at lower proportionate rates, than a number of shorter lines, each having a different set to salary, while the delay and vexation which not unfrequently arise from short routes, being compelled to wait upon each other's movements, will all be avoided, which is certainly no small consideration both to passengers and shippers.

The harbor of Portland is one of the finest and most eligible in the world, and our immediate connection with a point of such importance is of itself a matter deserving particular mention. Portland district, as appears by the official statement of the tonnage of the United States, made to June, 1857, then owned 145,242 tons of shipping, being the ninth port in the Union in point of tonnage; she is very largely interested in the West India trade, her annual imports of molasses exceeding those of any port in the United States. She offers, therefore, to the Western States, peculiar facilities for procuring at a cheap rate the products of the West Indies. The harbor is without any bar, and so easy of access that no pilots are required, and strangers, with the sailing directions given in the American Coast Pilot, have brought their ships into it with safety. There are no port charges, harbor dues, or light-house fees, excepting the official custom house fees.

The Grand Trunk Railway is likely to become the avenue through which an immense tide of immigration will pour into Michigan. It will be a favorite route for emigrants, who will thus avoid the rascally impositions of the swindlers and Peter Funks of New York, who have given that city an unenviable notoriety throughout the world. It is predicted that more immigrants will hereafter come by the new route than by all others put together. There is no valid reason why this prediction should not prove strictly true. This is therefore a matter likely to be of vast importance to our State, with a large share of her territory as yet an unbroken wild, offering tempting inducements to the hardy settler.

The completion of this stupendous bond of connection between the Eastern and Western States, Canada and Europe, will render markets available which were before difficult of access, and enable far-distant countries to exchange their products at all seasons. The Grand Trunk may be called the first section of the Pacific Railroad, as it already communicates with the Mississippi through Michigan, Illinois and Wisconsin Railroads, and we expect to see the line completed from the Mississippi to California. It is not easy to form an estimate of the amount of traffic and intercourse that the 1,150 miles of Grand Trunk Railway will bring to Michigan and the neighboring States. A junction has been already formed with that model of western lines the Michigan Central by which freight and passengers reach Chicago and the numerous lines which diverge from that great commercial city. It is probable that another junction will be made with the Detroit and Milwaukee Railway by means of a branch from Port Huron to Owasso. In this case there will be a direct line across Michigan connecting with the Milwaukee railroads by the ferry across the lake, and penetrating into Iowa, Minnesota, Nebraska, and Oregon by lines which have not yet been traced on the railway maps of the United States.

The ostensible western terminus of this road is at Windsor, opposite our city, but it is practically as much a Detroit road as any that can be named. The connections with the other routes centering here is made by a number of ferry boats of the most staunch and powerful description. The receipts by this route of general merchandise consigned to the cities and points westward of us is immense, and it enjoys a large and growing local traffic.