Another circumstance we ought to have mentioned was our passing through a very long tunnel, called the Board Tree Tunnel, about 340 miles from Baltimore. This tunnel, after having fallen in, has only been repaired within the last two months. The history of this catastrophe, and of the mode of remedying it, forms quite an incident in the history of the railway, and shows with what resolution difficulties in this country are overcome. To reopen the tunnel it was clear would be a work of time, so Mr. Tyson resolved to run a new temporary railway for three miles over the mountain which had been tunnelled, and this was accomplished by 3000 men in ten days. We saw the place where this road had passed, and the zig-zag line by which the mountain was crossed. The road seems positively to overhang the precipice, and reminded me of a mountain pass in Switzerland--as, indeed, the whole of the road here does. Mr. Tyson himself drove the first train over, and he said his heart was in his mouth when, having got to the top, he saw the descent before him, and the engine and train on a precipice where the least _contretemps_ would have plunged the whole into the abyss below; but happily all went right, and till within the last two months this temporary road has been used. It was really quite frightful to look up and think a train could pass over such a place, the grade being 420 feet in a mile, or 1 in 121/2; but you will one day be able to form some idea of it, as a photograph was taken, and Mr. Tyson will give us a copy of it. This is certainly a wonderful country for great enterprises, and the Pennsylvania Central Railway, by which we contemplate recrossing the Alleghanies, is in some respects a still more remarkable undertaking, though the height at which the mountains are crossed on that line is not so great as that on the Baltimore and Ohio line, which, as I told you in my last, is at an elevation of 2700 feet. It was long supposed that such a feat could not be surpassed, but Mr. Tyson says that, encouraged by this, a railway now crosses the Tyrolean Alps at a somewhat higher level.