This company, MicroFasteners has #4 x 3/4" in black oxide 100pcs for $3.90 US. Their minimum shipping for international orders is $7.00.So, that's 100 screws for the price of 48 Atlas screws
In RR-Track, Component view displays the layout parts - the track count is 235 x 2 screws per track =700
I searched for sheet metal screws with black oxide coating suppliers in brazil with no luck - stainless only - very expensive
Use 1 screw per track until you get more screws to continue assembly - one at each joint