Update:
A new version has been uploaded, including bugfixes (see the opening post for details: the main bugs recently identified by users have, as far as I can discern, now all been fixed), and a new feature: improved debt handling. Isidoro's replacing patch is also fully updated to the latest version - thank you, Isidoro.
There are three essential features of the new system: (1) bankruptcy is now optional (and disabled by default); (2) debts accumulate interest (at a customisable rate; 10% per annum by default); and (3) there is a credit limit, beyond which players cannot buy new capital items. Interest is added monthly based on the total debt at month's end. There is not currently a graph showing historical interest payments, as this would require reversioning the save file, but this might well be added in the future. The credit limit is a varying amount, based on the player's ****ets and profits over the last year. The minimum (and starting) credit limit is 10,000 Simucredits. The credit limit is displayed in the finances window.
Capital expenditure can be made only within the credit limit. For example, if a player has a 100,000c bank balance and a 100,000c credit limit, the player can purchase capital items to the value of up to 200,000c. Beyond that, the player can purchase no further capital items until the cash balance or credit limit increases. Items that are placed in the map view will give an error message telling the player that the credit limit is exceeded if the player tries to purchase them in excess of the limit. Vehicles in the depot will show a new colour when the player cannot afford them: orange. (Red is still shown for vehicles that cannot be bought as part of the current convoy in any case). For the new vehicle replacement system, players can select orange items (as the resale value of the existing vehicles might enable the new ones to be bought, even if there is currently not enough in the bank), but the window will give the "That would exceed your credit limit" error message if the total net cost of the entire replacement cycle is in excess of the credit limit. The one exception to the credit limit is the remove tool: it can be used no matter how great the player's debt. This is to enable players who get into trouble to remove unprofitable parts of their network to get themselves out of the difficult.
The credit limit can optionally be disabled, reverting the game to the original behaviour. The credit limit system is also ignored in freeplay mode (although, strictly, freeplay mode is now redundant because the same effect can be had by altering the debt settings in simuconf.tab.).
There are some new simuconf.tab settings to customise each of the three features. An excerpt from the new simuconf.tab file shows the settings and the explanation for them:
# Insolvency and debt settings
#
# These settings allow what happens when the player runs out of money and goes
# into the red to be customised.
#
# "interest_rate_percent" is the annual interest rate (charged monthly) on all
# overdraft debt (i.e., on all negative account balances). It can be between 0
# and 255.
#
# "allow_bankruptsy" determines whether, when the player is deemed to have been
# insolvent for more than a certain period, the player should be declared
# bankrupt and the game over. 0 = no, 1 = yes.
#
# "allow_purchases_when_insolvent" determines whether, if the player's bank
# balance falls below the player's credit limit (shown in the finance window),
# the player will be unable to spend any new money on capital items (excluding
# bulldozing) until the player has come back within the credit limit again.
# 0 = no, 1 = yes.
#
# To revert to the behaviour of Simutrans standard, set interest_rate_percent to
# 0, set allow_bankruptsy to 1, and set allow_purchases_when_insolvent to 1.
interest_rate_percent = 10
allow_bankruptsy = 0
allow_purhcases_when_insolvent = 0
The reasoning behind this patch is that, with the current standard version of Simutrans, the handling of debt is a little too crude: players face the ostensibly extreme sanction of bankruptcy after only three months of debt (or three months of debt over a certain limit, in newer versions), but can easily byp**** it by closing the "new map" window and carrying on. Meanwhile, players are free to run up as much debt as they please. That uneasy combination leads to players who have put a great deal of effort into their games but who have ended up insolvent facing a stark choice between, on the one hand, accepting the bankruptcy and losing all of their work, or, on the other hand, just closing the window and continuing; but the latter will often feel very unsatisfying. The idea of the new system is to introduce a softer but more effective disincentivisation for insolvency, by not allowing players to buy new things after they become insolvent. Players can then continue, and try to turn their transportation company around (without immediately losing all of their work), or sink into the mire of inescapable debt if they do very badly, and realise that starting again would be the best option all around. From a psychological and gameplay perspective, my view is that the new system is more satisfying. The interest is also there to introduce an element of cost to maintaining the debt: without it, there would be no point in having a separate credit limit, since the player's effective cash would be the player's actual cash plus the credit limit.
As ever, feedback (including Simutrans-Experimental specific bug reports) is always welcome.