I'm not sure that this is quite right! It is a fact that originally Trafalgar Street was the main access street to the station, and very steep. However my understanding is that originally when the station was smaller, the Goods Yard was on the west side, and at the same level as the main line. As the town and hence station expanded, the Goods Yard was moved to the east side, and at a lower level. It was for this reason that the small tunnel was built under the main platform: to get heavy goods from the east lower side to the higher main level. I don't doubt that it was also used to bring passengers in from the Trafalgar Street side of town. Also I doubt that it was even in existence, never mind closed, in 1845, just 4 years after the original station opened. I couldn't say when it was built, however check out the Station Tour with Jackie Marsh-Hobbs, Brighton Festival, she will know. I'm sure it was still in regular use all the time that horse drawn transportation was in use, right up to the early 1900's and probably only stopped being used when powerful motorised transport finally superseded four-legged transport!