A T-1 provides 1.536Mbps of bandwidth, total. It can be used fully up or down, or at any feasible combination of speeds in between but always maxing out at 1.536Mbps due to the 8kbps framing bit used to keep timing on the circuit. Here's a few references:
This is AT&T's spec for the origination of a T-1 circuit:
TR62411a: ACCUNET® T1.5 Service Description And Interface
220.127.116.11.1 Transmission Line Rate The electrical signals delivered to the Network and to the CI at the NI, must be T1 - type bipolar, except
where intentional bipolar variations are introduced by B8ZS (see
Section 4.2.2), operating at the nominal DS1 rate of 1.544 Mb/s ± 50
b/s (11). 11: Older equipment has rate variation up to +-200bps
5.6 64 kb/s Clear Channel Capability (64CCC) Clear Channel Capability is a characteristic of a DS1 transmission path in which the 192
information bits in a frame can represent any combination of zeros and
ones. Clear Channel Capability, as an option for ACCUNET T1.5 Service
customers, will be implemented exclusively through use of B8ZS line
encoding both on the access and interoffice channel (IOC) segments,
thus providing end-to-end B8ZS Clear Channel Capability. Thus, 64CCC
implemented through B8ZS, provides users with 64 kb/s of channel
capability, and on properly configured DS1 circuits, allows 1.536 Mb/s
of data transmission. (See related discussions on B8ZS and Pulse
Density restriction in Sections 4.2.2 and 5.5.)
In today's technological age there are T-1 approximate circuits, where they are providing you T-1 speed and guarantees, but might not be using the actual T-1 technology.
An example would be CenturyLink's Private Line service, defined as:
Private Line: T1/DS1 transmits of isochronous serial data at
1.544Mbps. The service has the capacity of 24 voice-equivalent channels.
The bolded part is the clarity some may be looking for. There are 24 bi-directional 64kbps channels on a T-1, and an 8kbps framing bit for timing.
24 channels * 64kbps each = 1536kbps + 8kbps = 1.544Mbps total CAPACITY.