An efficient but accurate beam dynamics model for linear accelerators has been developed and is being used in our control rooms, and in particular to commission the electron linear accelerator (e-linac). Other labs use simulations of up to ond million particles and then distill these down to only the 3 size parameters of the beam bunches. These multi-particle simulations are too slow to be used in online tuning of an accelerator. We have developed a technique that tracks bunch sizes, including space charge, instead of individual particles. Such applications have existed for many years but only for beam transport, not for linear accelerators. Our application allows operators to calculate new linac tunes online. It has generated international interest and an invited talk at the 2016 Linac Conference.