If you do a Google search for "BNSF Roster" you'll be taken to a number of websites that have information on BNSF's complete locomotive roster.
Here's a roster put out by Trains magazine
BNSF, like the other railroads, operate a vast mix of locomotives of varying ages. Most of the mainline power consists of GE Dash 9-44CW and successor models manufactured from the mid-1990s, with EMD SD70MACs primarily in coal service manufactured from 1993 onwards.
BNSF still has lots of 1970s era SD40-2s (many stored or retired), and local/yard power is still predominately GP38-2 and similar models originally built in the mid-to-late 1970s, along with fleets of GP39s that were rebuilt for BN in the late 1980s/early 1990s using GP30 and GP35 carbodies.
The oldest locomotive in the fleet appears to be three SD9s manufactured between 1954 and 1957 and have dash-3 electrical systems (and presumably 645 power assemblies); and nine SW1200s built 1955-1959 from the Burlington, C&S, NP and GN.