function trainwavesleft
X=0:0.01:20;
f = 5.25;
P = 1/f;
w = 2*pi*f;
v = 10;
k = w/v;
A = 1;
R = A*exp((1i*k)*X);
zeta1 = k*X-pi/2;
zeta2 = k*X+10.5*pi;
dt = P/8;
iw = -1i*w;
t = 0;
pause on
for n=1:140
    t = t + dt;
    Z = 1./(1+exp(20*(zeta1-w*t)));
    Z = Z./(1+exp(-20*(zeta2-w*t)));
    T = exp(iw*t);
    Y = real(R.*T);
    plot(X,Y.*Z)
    axis image
    axis([0 20 -2 2])
    axis off
    pause(0.1)
end;
pause off