Engine maintenance gets very high very fast in RT3 and it pays to replace engines after 10-20 years in scenarios that last 40 years or more. In this scenario, however, I cut maintenance in half so that you don't have to replace the engines. My normal rule for replacing an engine is to subtract the annual maintenance cost when new from the current maintenance cost, then multiply that by the age of the engine or if less, the number of years left in the scenario. If that number is a lot greater than the cost of the engine, I replace it. So, if the engine cost $40k, and its last full year maintenance was $20k and its new maint cost was $9k, I should theoretically replace it if it was 6 years old - 11x6=66. The reason is that you will spend at least $66k extra in maintenance over the next 6 years on this engine, and you can buy a new one for only $40k. I did ignore interst, so in reality I probably would wait until 10 years to replace this engine, and probably even longer because it seems sacriligous to get rid of an engine before its 20 years old.
A more precise formula would be (maint current - maint new) x (years old or years to go) > (enginecostnew + (enginecostnew x interest) x years). If interest was 10%, then we get:
(20 - 9 ) x years > (40 + (40 x .1) x years)
11 x years > (40 +4 x years)
(11-4) x years > 40
7 x years > 40, or about 6 years!
You save at least $11k a year and pay only $4k in interest, for a net of $7k x 6 = $42k and that is greater than a new engine costs.
So, what if you can't buy that engine, and the engine you can (or want) to buy costs $120,000. I just ignore the cost of the actual replacement engine because it is probably worth the extra cost. In most situations, you would rather buy a Consolidation over a Baldwin because of the extra speed and hauling capacity.
So the final formula is ((last full year maint - new full year maint + (original engine cost x interest) x years of expected use > original engine cost
Aren't you sorry you asked
![Mr. Green :mrgreen:](./images/smilies/icon_mrgreen.gif)