This release is only possible because of help from @GWS.
A new function has been added that allows for limitations on the links of a station. The station has an attribute called "link," which now refers to a specific link limited to certain lines. For instance, link="A:x,C" reveals that the link to station A can run only on line "x" even if the stations claim to be using more than one common line. If more than one link is forbidden from operating on one particular line, the lines are separated by pipes like this link="A:x|y,C." The previous links will not change as there are no additional signs. The invalid id will trigger Map::Tube::Exception::InvalidStationLineId.
The function get_shortest_route() is updated to accept line limitations of individual links. As a result, a link becomes charged for using a link that travels on a line which is not included in its limitations.
get_next_stations() and get_linked_stations now accept an optional $line_name parameter, allowing users to inquire "what is specifically linked to this station on this line" in a way that enforces link-by-link restrictions, as opposed to simply returning all physically linked neighbors regardless of the line.
The preferred() method of Map::Tube::Route also considers link restrictions when determining the ride line(s) to display for each segment, in accordance with the cost logic of get_shortest_route() (verifiable via t/02-preferred.t on a map that employs the new syntax).
A flaw was fixed that caused an error to occur due to the reading of unset link restriction via chained hash dereferencing. Although this flaw did not affect routing results, it did lead to faulty deep object comparison in Test::Map::Tube's ok_map_routes(), which then resulted in passing spurious test failures on maps that do not even employ the new syntax (stated in t/ok_map_other_link.t and t/ok_map_common_lines.t).