Files with the .tng file extension can contain all kinds of data
related to the trajectory of a simulation. For example, it might
contain coordinates, velocities, forces and/or energies. Various .mdp
file options control which of these are written by mdrun, whether data
is written with compression, and how lossy that compression can be.
This file is in portable binary format an can be read
with gmx dump.
% gmx dump -f traj.tng
or if you're not such a fast reader:
% gmxdump -f traj.tng | more
You can also get a quick look in the contents of the file (number of
frames etc.) using:
% gmx check -f traj.tng