Nobody knew the exact metallurgical condition of the bridge before the fire, but it did not seem to matter because every trip over the bridge added to the mountain of empirical evidence that the bridge was safe.
It may still be safe today, but the fire ruined the empirical evidence, so now we need a test to prove that it is safe. We can put men on the moon and shoot down incoming rockets, and build nuclear power plants, so we ought to be able to test a bridge. It is only a matter of cost. Is it cheaper to build a new bridge than it is to test the old one and save it?
The pattern of burning and scorching of the paint on the old bridge should be a good indication of the degree and location of heating from the fire. I suggest hiring a testing company to take core samples from various locations that represent the range of heating as well as areas where no significant heating occurred. Then lab test the samples to find out the following:
1) The material analysis, type of iron, hardness, fatiguing, etc.
2) The effect of the fire, if any.
3) The consistency of the material between different areas of the same heating temperature, and between different areas where no heating occurred.
If the test results show that the material is high quality, consistent, and un-affected by the fire heating, then simply re-deck the trestle and put it back in service.
If the results show some inconsistency in the material that was not heated by the fire, but no overall difference from the area that was heated by the fire, then probably re-deck the trestle and put it back in service.
If the results show fatigue, cracking, embrittlement, or other defects in the fire heated areas, but not in the non-fire heated areas such as the towers, then consider partial replacement and/or strengthening of components as needed; or build a completely new bridge.
This material sampling and analysis, coupled with a structural calculation of the bridge members, would tell the whole story of the existing condition and capacity. Not only would it show the effect of the fire, if any, but it would also show the precise underlying condition during the period of operation before the fire. That is something that has never been known. So that information would put to rest any concern going forward about the bridge being too old, worn out, or made of unpredictable material. It would end the previous need to assume the bridge is safe only because it has not collapsed under load.
As others have mentioned, while new bridges may be pretty enough and surely dispel any concerns about reliability and safety, they are out of context with the historical fabric of the trains and railroad infrastructure. It would be a shame to lose that authentic, historical bridge just because of unfounded worries about its strength after the fire. The facts of its condition may indicate that it is perfectly good for another century of operation.
