Solving Linear Equations on the Train

From a post by Alexandre Borovik at Mathematics under the Microscope: 

... from a mathematical point of view, solving a (elementary level) Sudoku puzzle is nothing more than solving a triangle system of Boolean equations by back substitution, something very similar to what we do after a Gauss-Jordan elimination in a system of simultaneous linear equations. But has anyone ever seen people on a train solving systems of linear equations from a newspaper?  ...

(As it happens, I have myself solved systems of linear equations on trains.  It was when I took some maths courses with the Open University 10 years ago.  I did most of my studying on the train to and from work.)