To avoid confusion, I'm talking about: "A flash of light is given off at the center of the traincar just as the two observers pass each other. The observer onboard the train sees the front and back of the traincar at fixed distances from the source of light and as such, according to this observer, the light will reach the front and back of the traincar at the same time." So assume the train has no windows and the onboard observer doesn't know which is the front or back of the train. Also, the light bulb is directly above the onboard observer's head when it is turned on. The observer has 3 sensors that detect the color (i.e. wavelength of light). He puts one sensor at each end of the inside of the train car. He puts the third sensor on top of his head (although I'm not really sure this is necessary for my question). My question is, will the sensors all report different wavelengths of light due to redshift? And if so, based on these differences, wouldn't the onboard observer be able to determine not only his direction of travel, but also his velocity, relative to the light source?