This answer is actually impossible to ascertain accurately. Figures on the sums of money the government gave and the land bequeathed to the Central and Union Pacific railroads ARE recorded, but in the fast and loose 1850's employment figures have a high chance of being lost, doctored or flat out innacurate. If someone could track the orders for rail to through shipment receipts or orders originating at the steel mills back east would be able to get a rough, but not exact estimation as track ordered was to extend the entire physical plants of both the UP and CP. Lumber for railroad ties in the original line weren't creosote treated and could have come from innumerable lineside lumbermill interests throughout California and the west. Explosives would probably be the most probable thing that could be traced. Obviously locomotive and rolling stock (freight abd passenger car orders) are fairly well recorded and thanks to a few pioneering photographers of the era, very well documented. You could use equations to guesstimate how much rail, ties and spikes were roughly used, but the harried pace both railroads built the barely usable original alignment varied wildely in tie spacing.

I'm on mobile, so I don't have the exact books an resources in front of me, but reasonably good reading on the subject is surprisingly hard to find considering the sheer VOLUME of books produced over the years has apocryphal facts and figures and no one source produced has taken an exhaustive look into accurate hard numbers.

Also, we are talking about the Pacific Railroad, and with that you should also take into account the ENORMOUS amount of bribery and graft that also factored into the cost, and the true scope of that died with each baron and politician.

TL;DR no spreadsheets, mo' problems.