Heading to Robstown, TX from Corpus Christi I saw a couple of light engines heading back to CC, and in Robstown a six engine Mexico bound mixed freight with some interesting power. These are in my Trainboard album as of 12/12/07. Here is a sample.
Wait a minute... what is 3490? At first I thought "C30-S7". Then I realized its rear section looked just like a Dash 8, so then I thought "C30-S7N". (The engine in front of it is a C30-S7.) Then I noticed the 8 full-height doors - it's a C40-8! But no C40-8s were delivered new to Mexico.
Actually, I think that says "MP". I believe this could be a C30-S7MP. Probably old 2300 or 2400 series from TFM.