The laws of Shabbat are derived from the various activities of the Tabernacle. The source for the laws of transporting from one domain to another is the case of loading the beams of the Tabernacle on wagons when the Children of Israel had to move. We learned in the Mishna (daf, 96a) that:
If there are two balconies [gezuztra’ot] that are private domains opposite each other on either side of the public domain, one who passesor throws an object from the one on this side to the one on that side is exempt. However, if the balconies were on the same level on the same side of the public thoroughfare, and the public domain separated the two, one who passes from one to the other is liable, and one who throws is exempt, as that method, passing, was the service of the Levites who carried the beams of the Tabernacle.
The Gemara on today’s daf discusses the service of the Levites carrying the beams by means of special wagons built for this purpose.
Rava said: The area on the sides of the wagon between the wagon and the wheel and the thickness of the wheel together equaled the full width of the wagon (Tosafot). And how much was the width of the wagon? It was two and a half cubits. The Gemara asks: Why do I need the wagon to be two and a half cubits wide? A cubit and a half would suffice. The Gemara answers: So that the beams would not teeter. Ten-cubit beams on a one-and-a-half-cubit wide surface would be unstable.
Each wagon, i.e., the frame on which the beams were placed, was two and a half cubits wide, and its wheel mechanism was two and a half cubits wide on either side. According to these measurements, each wagon took up a space of seven and a half cubits in width, and two wagons side by side, according to the opinion that beams were laid across two wagons, were, therefore, fifteen cubits in width. There was an additional cubit for the Levite who accompanied the wagon.
According to most of the commentaries, the wagons traveled two abreast, with some beams supported by both wagons. This is not at all similar to the halakha of two balconies. In that case liability is incurred only when passing from one balcony to another along the length of the public domain and not across its width. According to Rav Hai Gaon, there is no real difference between the length and width of the public domain. Rather, the halakha is derived from the Levites’ wagons. Wherever there are two private domains capable of being aligned on a single level, as in the case of the Levites’ wagons, the halakha prohibiting passing applies. On the other hand, the Rashba states: Since there is no way to join two balconies on either side of a public domain, the halakha prohibiting passing does not apply. The Me’iri explains that in order to expedite the process of loading the wagons, the Levites passed the beams from one wagon to another that was positioned behind it, similar to the case of balconies on the same side of the public domain.