[Note 336.1] Byron was hugely lionised during the early 19th century. The Dictionary of National Biography says that "Byron became a legend after his death. His influence on the art, music, and literature of the nineteenth century can scarcely be calculated"
[Note 336.2] When this song was published in 1846, railway share prices had already fallen from their 1845 peak and it was clear to many that were likely to fall further. (See notes on Bar 327, Railway Calls)
[Note 336.3] Parliament approved 3,000 miles of new railway during 1845 and a further 4,500 miles during 1846. The situation is illustrated in these two graphs from Freeman and Aldecroft's Atlas of British Railway History.
[Note 336.4] Naive investors drawn in and sold duff schemes sold on basis of extravagant or even fraudulent promises.
[Note 336.5] The railway was adopted enthusiasticly by ordinary people. This new mobility of the working classes was the cause of much comment and no little anxiety among the upper classes.
[Note 336.6] "Boxes" in this context almost certainly means "cases of larger size, made to hold merchandise and personal property" (OED). Given the original meaning of "Portmanteau" (see glossary) this interpretation of "boxes" is supported by the next line "So leaves off the portmanteau line and takes unto the trunk line". The railways quickly took over the long distance freight business. (See Theme 005, The Impact of Railways on Horse Powered Transport).
[Note 336.7] Railways carried horses and coaches on flat wagons enabling the gentry to travel in privacy. In 1839 the London & Brimingham announced that "gentlemen's carriages and horses must be at the station at least a quarter of an hour before time of departure". (Ref. The Grand Experiment : The Birth of the Railway Age 1820 - 1845)
[Note 336.8] As well as wildly overstating the likely profitability of the finished line, the proposals for new railways often seriously underestimated construction costs with the result that insufficient funds were available for labour and materials. In the year that this song was published, Brunel complained his costs had risen by 50% in one year. (Ref. The Grand Experiment : The Birth of the Railway Age 1820 - 1845)
[Note 336.9] Brunel was so forceful in his demands for the levelest possible track bed that the Great Western line came to be known as "Brunel's billiard table" (ref: The Grand Experiment : The Birth of the Railway Age 1820 - 1845). "Flats" in this context is a play on words referring to both the levelness of the track bed and the use of flat to mean a naive speculator.
The British Library catalogue lists several comic songs and theatrical pieces ascribed to James Bruton and published about the time of this song.