The term Trunk Line in telecommunications refers to the high-speed connection between telephone central offices in the Public Switched Telephone Network (PSTN). Trunk lines are always digital. The wiring between central offices was originally just pairs of twisted copper wire (the twists in the wiring prevented things known as crosstalk and noise). Because it is expensive to string up (or lay trenches for buried cables), the phone company researched ways in which to carry more data over the existing copper lines. This was achieved by using time-division multiplexing. Later, when fiber-optic technology became available, phone companies upgraded their trunk lines to fiber optics and used statistical time-division multiplexing, synchronous digital heirarchy, coarse or dense wave division multiplexing and optical switching to further improve transmission speeds.
NOTE: Keep in mind that a signal travelling over a trunk line is not actually flowing any faster. The electrical signal on a voice line takes the same amount of time to traverse the wire as a similar length trunk line. What makes trunk lines faster is that the signal has been altered to carry more data in less time using more advanced multiplexing and modulation techniques. If you compared a voice line and a trunk line and put them side by side and observed them, the first pieces of information arrive simultaneously on both the voice and trunk line. However, the last piece of information would arrive sooner on the trunk line. No matter what, you can't break the laws of physics. Electricity over copper or laser light over fiber optics, you cannot break the speed of light--though that has rarely stopped uneducated IT or IS managers from demanding that cabling perform faster instead of upgrading equipment.
Trunk lines can contain thousands of simultaneous calls that have been combined using time-division multiplexing. These thousands of calls are carried from one central office to another where they can be connected to a de-multiplexing device and switched through digital access cross connecting switches to reach the proper exchange and local phone number.
There are various terms used for trunk lines that you will hear telephone people use. These terms are not quite synonymous as there are differences in the type of information carried by the trunk lines and what the trunk lines connect.