March 11 - April 2, 1868: From New York to San Francisco. Departed New York, March 11 on board the Henry Chauncy. A wooden-hulled sidewheeler of 2,656 tons built in New York in 1865, it was owned and operated by the Pacific Mail Steamship Company. At 25⁰ latitude, the Henry Chauncey was just below the tip of Florida. The Pacific Mail Steamship Company’s “upward bound” steamer, bound from Aspinwall (now Colón) on 15 March, “met and communicated with” the Henry Chauncey on 16 March, and arrived in New York on 22 March. Clemens and his fellow passengers left the Henry Chauncey at Aspinwall on the morning of 19 March and traveled across the Isthmus in one day, mainly by train, and boarded the Sacramento at Panama City. They arrived in San Francisco on 2 April .

Along with Mark Twain on this voyage was John P. Vollmer, who in a letter to Albert Bigelow Paine (December 26, 1911) described an incident that occurred on the train ride crossing the Isthmus.

Our voyage to Aspinall was pleasant enough, and there we boarded a train for Panama.
At Barbacoas (a station) we were slightly delayed by some bad track, and progress to the next station was prospectively slow. A number of passengers, for the sake of novelty, elected to be carried, for an additional consideration, in a peculiarly shaped chair placed on the back of natives to the next station ahead or to where our train should overtake them which happened just opposite Cierro Gigante, the highest point on the Isthmus from which Balboa first saw the Atlantic and Pacific in one glance.
Mark Twain was among them. His carrier was a native sturdy buck whose complete dress attire consisted of a breech clout of most simple construction.
They started off well in the rear of the procession leaving Major Bright and me in our seats watching their departure.
Shortly our train started and within ten minutes we came up with the rear of the carrier's procession. A moment later, we overtook and passed our illustrious co-traveler.
But what a spectacle! There he was perched in a chair on the back of his human ship of the Isthmus, his legs dangling in unison with each step of his carrier. As his position was such that he could look only backwards, he was unaware of the scene in front. There was Mr. Native tramping along, seemingly proud and conscious of the illustrious burden he was carrying.
But here I must draw the curtain partly. The breech clout had become unfastened from its starboard moorings, to which circumstance the "Buck" appeared completely oblivious. The female portion of our voyagers, looking out of the car windows, suddenly lost all interest in the scenery on that side of the road.
We soon again took on board our fellow travelers who had left us at Baracoas.
When we recovered our friend from his picturesque position, from merciful considerations we were induced to promise upon honor never, during his life, to describe that circumstance nor to illustrate it nor to allow it.
Altho many times was I tempted to add such an illustration of him to the rear end of his lengthy and famous procession of barkeepers in Tramp Abroad. (I think he got his inspiration of that procession from his exploit near Barbacoas.)
After that he was content to pursue the rest of his journey incognito. Fortunately few knew him and those seemed to have failed to catch sight of the show. No wonder he kept to good behavior. He realized what danger would result to his ambitions should he be caricatured accordingly.

(Davis, Chester L. 1967. “Letter to Paine from John P. Vollmer.” Twainian 26 (March-April): 3–4.)